diff --git a/mongoose.c b/mongoose.c
index ddb83d39..51f5a9f5 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -1420,7 +1420,7 @@ int mg_read(struct mg_connection *conn, void *buf, size_t len) {
 
   assert((conn->content_len == -1 && conn->consumed_content == 0) ||
          conn->consumed_content <= conn->content_len);
-  DEBUG_TRACE(("%p %zu %lld %lld", buf, len,
+  DEBUG_TRACE(("%p %lu %lld %lld", buf, (unsigned long) len,
                conn->content_len, conn->consumed_content));
   nread = 0;
   if (conn->consumed_content < conn->content_len) {