diff --git a/src/userfunc.c b/src/userfunc.c
index 9216685..0f28b02 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -3199,6 +3199,12 @@
 	    p = vim_strchr(p, '(');
     }
 
+    if ((vim9script || eap->cmdidx == CMD_def) && VIM_ISWHITE(p[-1]))
+    {
+	semsg(_(e_no_white_space_allowed_before_str), "(");
+	goto ret_free;
+    }
+
     // In Vim9 script only global functions can be redefined.
     if (vim9script && eap->forceit && !is_global)
     {