diff --git a/lib/cf-socket.c b/lib/cf-socket.c
index 356b3657a4..b052781ffb 100644
--- a/lib/cf-socket.c
+++ b/lib/cf-socket.c
@@ -1141,7 +1141,7 @@ static ssize_t cf_socket_send(struct Curl_cfilter *cf, struct Curl_easy *data,
 
   DEBUGASSERT(data->conn == cf->conn);
   nwritten = Curl_send_plain(data, cf->sockindex, buf, len, err);
-  CF_DEBUGF(infof(data, CFMSG(cf, "send(len=%d) -> %d, err=%d"),
+  CF_DEBUGF(infof(data, CFMSG(cf, "send(len=%zu) -> %d, err=%d"),
             len, (int)nwritten, *err));
   return nwritten;
 }
@@ -1153,7 +1153,7 @@ static ssize_t cf_socket_recv(struct Curl_cfilter *cf, struct Curl_easy *data,
 
   DEBUGASSERT(data->conn == cf->conn);
   nread = Curl_recv_plain(data, cf->sockindex, buf, len, err);
-  CF_DEBUGF(infof(data, CFMSG(cf, "recv(len=%d) -> %d, err=%d"),
+  CF_DEBUGF(infof(data, CFMSG(cf, "recv(len=%zu) -> %d, err=%d"),
             len, (int)nread, *err));
   return nread;
 }