diff --git a/kittens/transfer/rsync.c b/kittens/transfer/rsync.c
index 966a83e25..550f1baff 100644
--- a/kittens/transfer/rsync.c
+++ b/kittens/transfer/rsync.c
@@ -49,7 +49,7 @@ iter_job(PyObject *self UNUSED, PyObject *args) {
     if (!PyArg_ParseTuple(args, "O!y#|p", &PyCapsule_Type, &job_capsule, &input_data, &input_data_size, &eof)) return NULL;
     GET_JOB_FROM_CAPSULE;
     if (eof == -1) eof = input_data_size > 0 ? 0 : 1;
-    rs_buffers_t buffer = {.avail_in=input_data_size, .next_in = input_data, .eof_in=eof, .avail_out=MAX(IO_BUFFER_SIZE, 2 * input_data_size)};
+    rs_buffers_t buffer = {.avail_in=input_data_size, .next_in = input_data, .eof_in=eof, .avail_out=MAX(IO_BUFFER_SIZE, 2 * (size_t)input_data_size)};
     PyObject *ans = PyBytes_FromStringAndSize(NULL, buffer.avail_out);
     if (!ans) return NULL;
     buffer.next_out = PyBytes_AS_STRING(ans);