diff --git a/src/calibre/utils/speedup.c b/src/calibre/utils/speedup.c
index 850c1f11c6..bdad0a7c6d 100644
--- a/src/calibre/utils/speedup.c
+++ b/src/calibre/utils/speedup.c
@@ -513,7 +513,7 @@ static PyObject*
 set_thread_name(PyObject *self, PyObject *args) {
 	(void)(self); (void)(args);
 #if defined(_MSC_VER)
-	PyErr_SetString(PyExc_OSError, "Setting thread names not supported on windows");
+	PyErr_SetString(PyExc_RuntimeError, "Setting thread names not supported on Windows");
 	return NULL;
 #else
 	char *name;