diff --git a/src/basic/fd-util.c b/src/basic/fd-util.c
index d63f012ad5..1a873601b2 100644
--- a/src/basic/fd-util.c
+++ b/src/basic/fd-util.c
@@ -454,7 +454,7 @@ bool fdname_is_valid(const char *s) {
                         return false;
         }
 
-        return p - s < 256;
+        return p - s <= FDNAME_MAX;
 }
 
 int fd_get_path(int fd, char **ret) {
diff --git a/src/basic/fd-util.h b/src/basic/fd-util.h
index f05c2b5a15..aa8e082b38 100644
--- a/src/basic/fd-util.h
+++ b/src/basic/fd-util.h
@@ -8,6 +8,9 @@
 
 #include "macro.h"
 
+/* maximum length of fdname */
+#define FDNAME_MAX 255
+
 /* Make sure we can distinguish fd 0 and NULL */
 #define FD_TO_PTR(fd) INT_TO_PTR((fd)+1)
 #define PTR_TO_FD(p) (PTR_TO_INT(p)-1)