diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index 4b39aae..b40e081 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -573,7 +573,7 @@
             Py_ssize_t old_size = PyBytes_GET_SIZE(self->unused_data);
             Py_ssize_t new_size;
             PyObject *new_data;
-            if (self->zst.avail_in > PY_SSIZE_T_MAX - old_size) {
+            if ((Py_ssize_t)self->zst.avail_in > PY_SSIZE_T_MAX - old_size) {
                 PyErr_NoMemory();
                 return -1;
             }