diff --git a/src/os_unix.c b/src/os_unix.c
index 11448c5..af8f006 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -6171,7 +6171,7 @@
     else if (STRCMP(how, "winch") == 0)
 	sig = SIGWINCH;
 #endif
-    else if (isdigit(*how))
+    else if (SAFE_isdigit(*how))
 	sig = atoi((char *)how);
     else
 	return FAIL;