diff --git a/src/testing.c b/src/testing.c
index c2389bd..80596d4 100644
--- a/src/testing.c
+++ b/src/testing.c
@@ -824,6 +824,9 @@
 {
     garray_T	ga;
 
+    if (in_vim9script() && check_for_string_arg(argvars, 0) == FAIL)
+	return;
+
     prepare_assert_error(&ga);
     ga_concat(&ga, tv_get_string(&argvars[0]));
     assert_error(&ga);