diff --git a/blueprintcompiler/gir.py b/blueprintcompiler/gir.py
index 029fe58..459086e 100644
--- a/blueprintcompiler/gir.py
+++ b/blueprintcompiler/gir.py
@@ -721,7 +721,7 @@ class TemplateType(GirType):
             # we don't know the template type's interfaces, assume yes
             return True
         elif self.parent is None or isinstance(self.parent, ExternType):
-            return isinstance(other, Class)
+            return isinstance(other, Class) or isinstance(other, ExternType)
         else:
             return self.parent.assignable_to(other)