diff --git a/src/userfunc.c b/src/userfunc.c
index 1d96a35..39d8fb2 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2374,6 +2374,7 @@
     int		extra = 0;
     lval_T	lv;
     int		vim9script;
+    static char *e_function_name = N_("E129: Function name required");
 
     if (fdp != NULL)
 	CLEAR_POINTER(fdp);
@@ -2401,7 +2402,7 @@
     if (end == start)
     {
 	if (!skip)
-	    emsg(_("E129: Function name required"));
+	    emsg(_(e_function_name));
 	goto theend;
     }
     if (end == NULL || (lv.ll_tv != NULL && (lead > 2 || lv.ll_range)))
@@ -2517,6 +2518,12 @@
 	}
 	len = (int)(end - lv.ll_name);
     }
+    if (len <= 0)
+    {
+	if (!skip)
+	    emsg(_(e_function_name));
+	goto theend;
+    }
 
     // In Vim9 script a user function is script-local by default.
     vim9script = ASCII_ISUPPER(*start) && in_vim9script();