diff --git a/lib/bup/_helpers.c b/lib/bup/_helpers.c
index 2790b07..b418900 100644
--- a/lib/bup/_helpers.c
+++ b/lib/bup/_helpers.c
@@ -1128,7 +1128,7 @@ static PyObject *write_idx(PyObject *self, PyObject *args)
 	PyList_Sort(part);
         uint32_t plen;
         if (!INTEGRAL_ASSIGNMENT_FITS(&plen, PyList_GET_SIZE(part))
-            || UINT32_MAX - count < plen) {
+            || UINT32_MAX - count < (uint32_t) plen) {
             PyErr_Format(PyExc_OverflowError, "too many objects in index part");
             goto clean_and_return;
         }