ctype_digit expects a string.

This function expects a string to be useful, so for example passing in
an integer may not return the expected result.

Author: lezama

class.jetpack-signature.php

@@ -121,7 +121,7 @@ function sign_request( $token = '', $timestamp = 0, $nonce = '', $body_hash = ''
 			}
 		}
 
-		if ( !ctype_digit( $timestamp ) || 10 < strlen( $timestamp ) ) { // If Jetpack is around in 275 years, you can blame mdawaffe for the bug.
+		if ( !ctype_digit( "$timestamp" ) || 10 < strlen( $timestamp ) ) { // If Jetpack is around in 275 years, you can blame mdawaffe for the bug.
 			return new Jetpack_Error( 'invalid_signature', sprintf( 'The required "%s" parameter is malformed.', 'timestamp' ) );
 		}