diff --git a/artiq/compiler/transforms/cast_monomorphizer.py b/artiq/compiler/transforms/cast_monomorphizer.py
index c12eb9663..bed905c5a 100644
--- a/artiq/compiler/transforms/cast_monomorphizer.py
+++ b/artiq/compiler/transforms/cast_monomorphizer.py
@@ -13,9 +13,10 @@ class CastMonomorphizer(algorithm.Visitor):
     def visit_CallT(self, node):
         self.generic_visit(node)
 
-        if (types.is_builtin(node.func.type, "int") or
-                types.is_builtin(node.func.type, "int32") or
-                types.is_builtin(node.func.type, "int64")):
+        if ((types.is_builtin(node.func.type, "int") or
+                    types.is_builtin(node.func.type, "int32") or
+                    types.is_builtin(node.func.type, "int64")) and
+                types.is_var(node.type)):
             typ = node.type.find()
             if (not types.is_var(typ["width"]) and
                     builtins.is_int(node.args[0].type) and