diff --git a/src/eval.c b/src/eval.c
index 19ab015..a9f7112 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -968,7 +968,7 @@
 	    // If the result is a string, check if there is a non-digit before
 	    // the number.
 	    s = tv.vval.v_string;
-	    if (!VIM_ISDIGIT(*s) && *s != '-')
+	    if (*s != NUL && !VIM_ISDIGIT(*s) && *s != '-')
 		*cp = *s++;
 	    retval = atol((char *)s);
 	}