diff --git a/src/calibre/utils/speedup.c b/src/calibre/utils/speedup.c
index c0f018ad17..400c1f7a4a 100644
--- a/src/calibre/utils/speedup.c
+++ b/src/calibre/utils/speedup.c
@@ -580,7 +580,9 @@ static PyMethodDef speedup_methods[] = {
 static int
 exec_module(PyObject *module) {
     PyDateTime_IMPORT;
+#ifndef _WIN32
     PyModule_AddIntConstant(module, "O_CLOEXEC", O_CLOEXEC);
+#endif
     return 0;
 }