Code had returned an ssize_t, upcast to long, then converted with PyInt_FromLong.
Now using PyInt_FromSsize_t.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index bcf2c38..aff14f5 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5306,7 +5306,7 @@
     if (end < 0)
         end = 0;
 
-    result = PyInt_FromLong((long) count(self, start, end, substring));
+    result = PyInt_FromSsize_t(count(self, start, end, substring));
 
     Py_DECREF(substring);
     return result;