diff --git a/pkg/nnbd_migration/lib/src/graph_builder.dart b/pkg/nnbd_migration/lib/src/graph_builder.dart
index 6f99c8a..fa47018 100644
--- a/pkg/nnbd_migration/lib/src/graph_builder.dart
+++ b/pkg/nnbd_migration/lib/src/graph_builder.dart
@@ -433,6 +433,9 @@
     } else {
       var typeArgumentType = _variables.decoratedTypeAnnotation(
           _source, node.typeArguments.arguments[0]);
+      if (typeArgumentType == null) {
+        _unimplemented(node, 'Could not compute type argument type');
+      }
       for (var element in node.elements) {
         if (element is Expression) {
           _handleAssignment(typeArgumentType, element);