diff --git a/src/ArduinoJsonJWT.h b/src/ArduinoJsonJWT.h
index fdaeb0b..f4263da 100644
--- a/src/ArduinoJsonJWT.h
+++ b/src/ArduinoJsonJWT.h
@@ -17,7 +17,7 @@ private:
   String _secret;
 
   const String JWT_HEADER = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9";
-  const size_t JWT_HEADER_SIZE = JWT_HEADER.length();
+  const int JWT_HEADER_SIZE = JWT_HEADER.length();
 
   String sign(String &value);