diff --git a/go/src/v.io/x/sensorlog/internal/sbmodel/sbmodel.vdl.go b/go/src/v.io/x/sensorlog/internal/sbmodel/sbmodel.vdl.go
index ff0f41a..de63374 100644
--- a/go/src/v.io/x/sensorlog/internal/sbmodel/sbmodel.vdl.go
+++ b/go/src/v.io/x/sensorlog/internal/sbmodel/sbmodel.vdl.go
@@ -44,28 +44,43 @@
 	if err != nil {
 		return err
 	}
-
 	keyTarget2, fieldTarget3, err := fieldsTarget1.StartField("Desc")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget3.FromString(string(m.Desc), tt.NonOptional().Field(0).Type); err != nil {
-			return err
+
+		var4 := (m.Desc == "")
+		if var4 {
+			if err := fieldTarget3.FromZero(tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget3.FromString(string(m.Desc), tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
 		}
 		if err := fieldsTarget1.FinishField(keyTarget2, fieldTarget3); err != nil {
 			return err
 		}
 	}
-	keyTarget4, fieldTarget5, err := fieldsTarget1.StartField("SgPublishSb")
+	keyTarget5, fieldTarget6, err := fieldsTarget1.StartField("SgPublishSb")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget5.FromString(string(m.SgPublishSb), tt.NonOptional().Field(1).Type); err != nil {
-			return err
+
+		var7 := (m.SgPublishSb == "")
+		if var7 {
+			if err := fieldTarget6.FromZero(tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget6.FromString(string(m.SgPublishSb), tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
 		}
-		if err := fieldsTarget1.FinishField(keyTarget4, fieldTarget5); err != nil {
+		if err := fieldsTarget1.FinishField(keyTarget5, fieldTarget6); err != nil {
 			return err
 		}
 	}
@@ -115,6 +130,10 @@
 
 	return nil
 }
+func (t *VDeviceCfgTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = VDeviceCfg{}
+	return nil
+}
 
 type KDeviceCfg struct {
 	DevId string
@@ -130,14 +149,21 @@
 	if err != nil {
 		return err
 	}
-
 	keyTarget2, fieldTarget3, err := fieldsTarget1.StartField("DevId")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget3.FromString(string(m.DevId), tt.NonOptional().Field(0).Type); err != nil {
-			return err
+
+		var4 := (m.DevId == "")
+		if var4 {
+			if err := fieldTarget3.FromZero(tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget3.FromString(string(m.DevId), tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
 		}
 		if err := fieldsTarget1.FinishField(keyTarget2, fieldTarget3); err != nil {
 			return err
@@ -184,6 +210,10 @@
 
 	return nil
 }
+func (t *KDeviceCfgTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = KDeviceCfg{}
+	return nil
+}
 
 // Sampling script and polling frequency.
 type SamplerDef struct {
@@ -205,52 +235,75 @@
 	if err != nil {
 		return err
 	}
-
 	keyTarget2, fieldTarget3, err := fieldsTarget1.StartField("Script")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget3.FromString(string(m.Script), tt.NonOptional().Field(0).Type); err != nil {
-			return err
+
+		var4 := (m.Script == "")
+		if var4 {
+			if err := fieldTarget3.FromZero(tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget3.FromString(string(m.Script), tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
 		}
 		if err := fieldsTarget1.FinishField(keyTarget2, fieldTarget3); err != nil {
 			return err
 		}
 	}
-	var wireValue4 time_2.Time
-	if err := time_2.TimeFromNative(&wireValue4, m.Start); err != nil {
+	var wireValue5 time_2.Time
+	if err := time_2.TimeFromNative(&wireValue5, m.Start); err != nil {
 		return err
 	}
 
-	keyTarget5, fieldTarget6, err := fieldsTarget1.StartField("Start")
+	keyTarget6, fieldTarget7, err := fieldsTarget1.StartField("Start")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
 
-		if err := wireValue4.FillVDLTarget(fieldTarget6, tt.NonOptional().Field(1).Type); err != nil {
-			return err
+		var8 := (wireValue5 == time_2.Time{})
+		if var8 {
+			if err := fieldTarget7.FromZero(tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
+		} else {
+
+			if err := wireValue5.FillVDLTarget(fieldTarget7, tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
 		}
-		if err := fieldsTarget1.FinishField(keyTarget5, fieldTarget6); err != nil {
+		if err := fieldsTarget1.FinishField(keyTarget6, fieldTarget7); err != nil {
 			return err
 		}
 	}
-	var wireValue7 time_2.Duration
-	if err := time_2.DurationFromNative(&wireValue7, m.Interval); err != nil {
+	var wireValue9 time_2.Duration
+	if err := time_2.DurationFromNative(&wireValue9, m.Interval); err != nil {
 		return err
 	}
 
-	keyTarget8, fieldTarget9, err := fieldsTarget1.StartField("Interval")
+	keyTarget10, fieldTarget11, err := fieldsTarget1.StartField("Interval")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
 
-		if err := wireValue7.FillVDLTarget(fieldTarget9, tt.NonOptional().Field(2).Type); err != nil {
-			return err
+		var12 := (wireValue9 == time_2.Duration{})
+		if var12 {
+			if err := fieldTarget11.FromZero(tt.NonOptional().Field(2).Type); err != nil {
+				return err
+			}
+		} else {
+
+			if err := wireValue9.FillVDLTarget(fieldTarget11, tt.NonOptional().Field(2).Type); err != nil {
+				return err
+			}
 		}
-		if err := fieldsTarget1.FinishField(keyTarget8, fieldTarget9); err != nil {
+		if err := fieldsTarget1.FinishField(keyTarget10, fieldTarget11); err != nil {
 			return err
 		}
 	}
@@ -305,6 +358,10 @@
 
 	return nil
 }
+func (t *SamplerDefTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = SamplerDef{}
+	return nil
+}
 
 // streamdef : <DevId>/<StreamId>
 // Configures a stream of data to be measured.
@@ -327,41 +384,64 @@
 	if err != nil {
 		return err
 	}
-
 	keyTarget2, fieldTarget3, err := fieldsTarget1.StartField("Desc")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget3.FromString(string(m.Desc), tt.NonOptional().Field(0).Type); err != nil {
-			return err
+
+		var4 := (m.Desc == "")
+		if var4 {
+			if err := fieldTarget3.FromZero(tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget3.FromString(string(m.Desc), tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
 		}
 		if err := fieldsTarget1.FinishField(keyTarget2, fieldTarget3); err != nil {
 			return err
 		}
 	}
-	keyTarget4, fieldTarget5, err := fieldsTarget1.StartField("Sampler")
+	keyTarget5, fieldTarget6, err := fieldsTarget1.StartField("Sampler")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
 
-		if err := m.Sampler.FillVDLTarget(fieldTarget5, tt.NonOptional().Field(1).Type); err != nil {
-			return err
+		var7 := (m.Sampler == SamplerDef{})
+		if var7 {
+			if err := fieldTarget6.FromZero(tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
+		} else {
+
+			if err := m.Sampler.FillVDLTarget(fieldTarget6, tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
 		}
-		if err := fieldsTarget1.FinishField(keyTarget4, fieldTarget5); err != nil {
+		if err := fieldsTarget1.FinishField(keyTarget5, fieldTarget6); err != nil {
 			return err
 		}
 	}
-	keyTarget6, fieldTarget7, err := fieldsTarget1.StartField("Enabled")
+	keyTarget8, fieldTarget9, err := fieldsTarget1.StartField("Enabled")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget7.FromBool(bool(m.Enabled), tt.NonOptional().Field(2).Type); err != nil {
-			return err
+
+		var10 := (m.Enabled == false)
+		if var10 {
+			if err := fieldTarget9.FromZero(tt.NonOptional().Field(2).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget9.FromBool(bool(m.Enabled), tt.NonOptional().Field(2).Type); err != nil {
+				return err
+			}
 		}
-		if err := fieldsTarget1.FinishField(keyTarget6, fieldTarget7); err != nil {
+		if err := fieldsTarget1.FinishField(keyTarget8, fieldTarget9); err != nil {
 			return err
 		}
 	}
@@ -416,6 +496,10 @@
 
 	return nil
 }
+func (t *VStreamDefTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = VStreamDef{}
+	return nil
+}
 
 type KStreamDef struct {
 	DevId    string
@@ -432,28 +516,43 @@
 	if err != nil {
 		return err
 	}
-
 	keyTarget2, fieldTarget3, err := fieldsTarget1.StartField("DevId")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget3.FromString(string(m.DevId), tt.NonOptional().Field(0).Type); err != nil {
-			return err
+
+		var4 := (m.DevId == "")
+		if var4 {
+			if err := fieldTarget3.FromZero(tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget3.FromString(string(m.DevId), tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
 		}
 		if err := fieldsTarget1.FinishField(keyTarget2, fieldTarget3); err != nil {
 			return err
 		}
 	}
-	keyTarget4, fieldTarget5, err := fieldsTarget1.StartField("StreamId")
+	keyTarget5, fieldTarget6, err := fieldsTarget1.StartField("StreamId")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget5.FromString(string(m.StreamId), tt.NonOptional().Field(1).Type); err != nil {
-			return err
+
+		var7 := (m.StreamId == "")
+		if var7 {
+			if err := fieldTarget6.FromZero(tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget6.FromString(string(m.StreamId), tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
 		}
-		if err := fieldsTarget1.FinishField(keyTarget4, fieldTarget5); err != nil {
+		if err := fieldsTarget1.FinishField(keyTarget5, fieldTarget6); err != nil {
 			return err
 		}
 	}
@@ -503,6 +602,10 @@
 
 	return nil
 }
+func (t *KStreamDefTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = KStreamDef{}
+	return nil
+}
 
 // sdata : <DevId>/<StreamId>/<Timestamp>
 // Measured data value or error.
@@ -522,46 +625,69 @@
 	if err != nil {
 		return err
 	}
-
 	keyTarget2, fieldTarget3, err := fieldsTarget1.StartField("DevId")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget3.FromString(string(m.DevId), tt.NonOptional().Field(0).Type); err != nil {
-			return err
+
+		var4 := (m.DevId == "")
+		if var4 {
+			if err := fieldTarget3.FromZero(tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget3.FromString(string(m.DevId), tt.NonOptional().Field(0).Type); err != nil {
+				return err
+			}
 		}
 		if err := fieldsTarget1.FinishField(keyTarget2, fieldTarget3); err != nil {
 			return err
 		}
 	}
-	keyTarget4, fieldTarget5, err := fieldsTarget1.StartField("StreamId")
-	if err != vdl.ErrFieldNoExist && err != nil {
-		return err
-	}
-	if err != vdl.ErrFieldNoExist {
-		if err := fieldTarget5.FromString(string(m.StreamId), tt.NonOptional().Field(1).Type); err != nil {
-			return err
-		}
-		if err := fieldsTarget1.FinishField(keyTarget4, fieldTarget5); err != nil {
-			return err
-		}
-	}
-	var wireValue6 time_2.Time
-	if err := time_2.TimeFromNative(&wireValue6, m.Timestamp); err != nil {
-		return err
-	}
-
-	keyTarget7, fieldTarget8, err := fieldsTarget1.StartField("Timestamp")
+	keyTarget5, fieldTarget6, err := fieldsTarget1.StartField("StreamId")
 	if err != vdl.ErrFieldNoExist && err != nil {
 		return err
 	}
 	if err != vdl.ErrFieldNoExist {
 
-		if err := wireValue6.FillVDLTarget(fieldTarget8, tt.NonOptional().Field(2).Type); err != nil {
+		var7 := (m.StreamId == "")
+		if var7 {
+			if err := fieldTarget6.FromZero(tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
+		} else {
+			if err := fieldTarget6.FromString(string(m.StreamId), tt.NonOptional().Field(1).Type); err != nil {
+				return err
+			}
+		}
+		if err := fieldsTarget1.FinishField(keyTarget5, fieldTarget6); err != nil {
 			return err
 		}
-		if err := fieldsTarget1.FinishField(keyTarget7, fieldTarget8); err != nil {
+	}
+	var wireValue8 time_2.Time
+	if err := time_2.TimeFromNative(&wireValue8, m.Timestamp); err != nil {
+		return err
+	}
+
+	keyTarget9, fieldTarget10, err := fieldsTarget1.StartField("Timestamp")
+	if err != vdl.ErrFieldNoExist && err != nil {
+		return err
+	}
+	if err != vdl.ErrFieldNoExist {
+
+		var11 := (wireValue8 == time_2.Time{})
+		if var11 {
+			if err := fieldTarget10.FromZero(tt.NonOptional().Field(2).Type); err != nil {
+				return err
+			}
+		} else {
+
+			if err := wireValue8.FillVDLTarget(fieldTarget10, tt.NonOptional().Field(2).Type); err != nil {
+				return err
+			}
+		}
+		if err := fieldsTarget1.FinishField(keyTarget9, fieldTarget10); err != nil {
 			return err
 		}
 	}
@@ -616,6 +742,10 @@
 
 	return nil
 }
+func (t *KDataPointTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = KDataPoint{}
+	return nil
+}
 
 type (
 	// VDataPoint represents any single field of the VDataPoint union type.
@@ -749,6 +879,10 @@
 
 	return nil
 }
+func (t *VDataPointTarget) FromZero(tt *vdl.Type) error {
+	*t.Value = VDataPoint(VDataPointValue{})
+	return nil
+}
 
 type vDataPointTargetFactory struct{}
 
@@ -759,17 +893,6 @@
 	return nil, fmt.Errorf("got %T, want *VDataPoint", union)
 }
 
-// Create zero values for each type.
-var (
-	__VDLZeroVDeviceCfg = VDeviceCfg{}
-	__VDLZeroKDeviceCfg = KDeviceCfg{}
-	__VDLZeroSamplerDef = SamplerDef{}
-	__VDLZeroVStreamDef = VStreamDef{}
-	__VDLZeroKStreamDef = KStreamDef{}
-	__VDLZeroKDataPoint = KDataPoint{}
-	__VDLZeroVDataPoint = VDataPoint(VDataPointValue{})
-)
-
 var __VDLInitCalled bool
 
 // __VDLInit performs vdl initialization.  It is safe to call multiple times.
@@ -789,6 +912,7 @@
 	if __VDLInitCalled {
 		return struct{}{}
 	}
+	__VDLInitCalled = true
 
 	// Register types.
 	vdl.Register((*VDeviceCfg)(nil))