diff --git a/lib/cw-out.c b/lib/cw-out.c
index 15857decfc..31d6b8f8b5 100644
--- a/lib/cw-out.c
+++ b/lib/cw-out.c
@@ -228,7 +228,7 @@ static CURLcode cw_out_ptr_flush(struct cw_out_ctx *ctx,
   curl_write_callback wcb = NULL;
   void *wcb_data;
   size_t max_write, min_write;
-  size_t wlen, nwritten;
+  size_t wlen, nwritten = 0;
   CURLcode result = CURLE_OK;
 
   /* If we errored once, we do not invoke the client callback again */