diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 65e1826..5aa67dd 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -234,6 +234,11 @@
                             "timeout period too long");
             return NULL;
         }
+        if (timeout < 0) {
+            PyErr_SetString(PyExc_ValueError,
+                        "timeout must be non-negative");
+            return NULL;
+        }
         seconds = (long)timeout;
         timeout = timeout - (double)seconds;
         tv.tv_sec = seconds;