diff --git a/src/eval.c b/src/eval.c
index b2673dc..abfde87 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2001,7 +2001,7 @@
     // Need to make a copy, in case evaluating the arguments makes
     // the name invalid.
     s = vim_strsave(s);
-    if (s == NULL || (flags & EVAL_CONSTANT))
+    if (s == NULL || *s == NUL || (flags & EVAL_CONSTANT))
 	ret = FAIL;
     else
     {