diff --git a/net.c b/net.c
index 3d0fde7..6aaa653 100644
--- a/net.c
+++ b/net.c
@@ -739,9 +739,9 @@ int qemu_find_nic_model(NICInfo *nd, const char * const *models,
 
 int net_handle_fd_param(Monitor *mon, const char *param)
 {
-    if (!qemu_isdigit(param[0])) {
-        int fd;
+    int fd;
 
+    if (!qemu_isdigit(param[0]) && mon) {
         fd = monitor_get_fd(mon, param);
         if (fd == -1) {
             error_report("No file descriptor named %s found", param);
@@ -750,7 +750,13 @@ int net_handle_fd_param(Monitor *mon, const char *param)
 
         return fd;
     } else {
-        return strtol(param, NULL, 0);
+        char *endptr = NULL;
+
+        fd = strtol(param, &endptr, 10);
+        if (*endptr || (fd == 0 && param == endptr)) {
+            return -1;
+        }
+        return fd;
     }
 }