diff --git a/terraform/context_plan_test.go b/terraform/context_plan_test.go
index 53bd8136c..f5eadd40b 100644
--- a/terraform/context_plan_test.go
+++ b/terraform/context_plan_test.go
@@ -2267,12 +2267,12 @@ func TestContext2Plan_computedMultiIndex(t *testing.T) {
 		case "aws_instance.foo[0]":
 			checkVals(t, objectVal(t, schema, map[string]cty.Value{
 				"ip":  cty.UnknownVal(cty.List(cty.String)),
-				"foo": cty.ListValEmpty(cty.String),
+				"foo": cty.NullVal(cty.List(cty.String)),
 			}), ric.After)
 		case "aws_instance.foo[1]":
 			checkVals(t, objectVal(t, schema, map[string]cty.Value{
 				"ip":  cty.UnknownVal(cty.List(cty.String)),
-				"foo": cty.ListValEmpty(cty.String),
+				"foo": cty.NullVal(cty.List(cty.String)),
 			}), ric.After)
 		case "aws_instance.bar[0]":
 			checkVals(t, objectVal(t, schema, map[string]cty.Value{
diff --git a/terraform/diff.go b/terraform/diff.go
index 58d5d9569..04cfaa0c7 100644
--- a/terraform/diff.go
+++ b/terraform/diff.go
@@ -497,7 +497,8 @@ func (d *InstanceDiff) blockDiff(path []string, attrs map[string]string, schema
 
 		// we can only trust the diff for sets, since the path changes, so don't
 		// count existing values as candidate keys. If it turns out we're
-		// keeping the
+		// keeping the attributes, we will check catch it down below with
+		// "keepBlock" after we check the set count.
 		if block.Nesting != configschema.NestingSet {
 			for k := range attrs {
 				if strings.HasPrefix(k, blockKey) {