diff --git a/src/vim9compile.c b/src/vim9compile.c
index 634f24e..6a3c32a 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -403,8 +403,12 @@
     if (ret == OK)
 	return OK;
 
+    // If actual a constant a runtime check makes no sense.  If it's
+    // null_function it is OK.
+    if (actual_is_const && ret == MAYBE && actual == &t_func_unknown)
+	return OK;
+
     // If the actual type can be the expected type add a runtime check.
-    // If it's a constant a runtime check makes no sense.
     if (!actual_is_const && ret == MAYBE && use_typecheck(actual, expected))
     {
 	generate_TYPECHECK(cctx, expected, offset, where.wt_index);