diff --git a/src/core/ngx_output_chain.c b/src/core/ngx_output_chain.c
index 4b8364a69..4479edacc 100644
--- a/src/core/ngx_output_chain.c
+++ b/src/core/ngx_output_chain.c
@@ -445,7 +445,7 @@ ngx_chain_writer(void *data, ngx_chain_t *in)
         size += ngx_buf_size(in->buf);
 
         ngx_log_debug1(NGX_LOG_DEBUG_CORE, ctx->connection->log, 0,
-                       "chain writer buf size: %uz", ngx_buf_size(in->buf));
+                       "chain writer buf size: %uO", ngx_buf_size(in->buf));
 
         cl = ngx_alloc_chain_link(ctx->pool);
         if (cl == NULL) {