helper/diff: handle unknownvariablevalue

helper/diff/resource_builder.go

@@ -3,6 +3,7 @@ package diff
 import (
 	"strings"
 
+	"github.com/hashicorp/terraform/config"
 	"github.com/hashicorp/terraform/flatmap"
 	"github.com/hashicorp/terraform/terraform"
 )
@@ -94,7 +95,7 @@ func (b *ResourceBuilder) Diff(
 
 			// If this key is in the cleaned config, then use that value
 			// because it'll have its variables properly interpolated
-			if cleanV, ok := flatConfig[k]; ok {
+			if cleanV, ok := flatConfig[k]; ok && cleanV != config.UnknownVariableValue {
 				v = cleanV
 				originalV = v