diff --git a/quiche/common/quiche_protocol_flags_list.h b/quiche/common/quiche_protocol_flags_list.h
index de2a763..41438b7 100644
--- a/quiche/common/quiche_protocol_flags_list.h
+++ b/quiche/common/quiche_protocol_flags_list.h
@@ -281,7 +281,7 @@
     "Alarm update granularity used by the QUICHE multiplexer alarm")
 
 QUICHE_PROTOCOL_FLAG(
-    size_t, quic_preallocate_unacked_packets, 0,
+    uint32_t, quic_preallocate_unacked_packets, 0,
     "Preallocate the specified number of entries in the unacked packet map.  "
     "If QuicConnection::GetUnackedMapInitialCapacity() already specifies a "
     "number, the larger of two applies.")