diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index de527ce..3973124 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1236,6 +1236,11 @@
                      1);
      */
     if (fileno != -1 && fileno != 0) {
+        /* Ensure that fileno is within the CRT's valid range */
+        if (_PyVerify_fd(fileno) == 0) {
+            PyErr_SetFromErrno(mmap_module_error);
+            return NULL;
+        }
         fh = (HANDLE)_get_osfhandle(fileno);
         if (fh==(HANDLE)-1) {
             PyErr_SetFromErrno(mmap_module_error);