diff --git a/src/cmd/compile/internal/types2/operand.go b/src/cmd/compile/internal/types2/operand.go
index 8336451e9ce..19326b83428 100644
--- a/src/cmd/compile/internal/types2/operand.go
+++ b/src/cmd/compile/internal/types2/operand.go
@@ -273,7 +273,7 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) (bool, er
 
 	// x is an untyped value representable by a value of type T.
 	if isUntyped(Vu) {
-		if t, ok := Tu.(*TypeParam); ok {
+		if t, ok := under(T).(*TypeParam); ok {
 			return t.is(func(t *term) bool {
 				// TODO(gri) this could probably be more efficient
 				if t.tilde {