diff --git a/src/eval.c b/src/eval.c
index 8563aa6..815d13d 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -4276,7 +4276,7 @@
 	return FAIL;
     end_leader = *arg;
 
-    if (**arg == '.' && (!isdigit(*(*arg + 1)) || in_old_script(2)))
+    if (**arg == '.' && (!SAFE_isdigit(*(*arg + 1)) || in_old_script(2)))
     {
 	semsg(_(e_invalid_expression_str), *arg);
 	++*arg;