diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index afce6c9..4166015 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -2429,7 +2429,8 @@
 /* The following requests n callbacks to _pending_callback.  It can be
  * run from any python thread.
  */
-PyObject *pending_threadfunc(PyObject *self, PyObject *arg)
+static PyObject *
+pending_threadfunc(PyObject *self, PyObject *arg)
 {
     PyObject *callable;
     int r;