diff --git a/src/userfunc.c b/src/userfunc.c
index d5dd369..c3b2a25 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3060,7 +3060,11 @@
 	clear_tv(rettv);
 	rettv->v_type = VAR_NUMBER;
 	rettv->vval.v_number = -1;
-	retval = FCERR_FAILED;
+
+	// In corner cases returning a "failed" value is not backwards
+	// compatible.  Only do this for Vim9 script.
+	if (in_vim9script())
+	    retval = FCERR_FAILED;
     }
 
 #ifdef FEAT_PROFILE