diff --git a/tls-client/main.cpp b/tls-client/main.cpp
index 1c4271330..7f5db927b 100644
--- a/tls-client/main.cpp
+++ b/tls-client/main.cpp
@@ -254,7 +254,7 @@ class HelloHTTPS {
         _bpos = snprintf(_buffer, sizeof(_buffer) - 1,
                          "GET %s HTTP/1.1\nHost: %s\n\n", path, HTTPS_SERVER_NAME);
 
-        int offset = 0;
+        size_t offset = 0;
         do {
             ret = mbedtls_ssl_write(&_ssl,
                                     (const unsigned char *) _buffer + offset,