Debug button doesn't become active when integer argument = 0

Hello team,
when I try to debug a function that has one or more integer arguments
using 0 as input the "Debug" button doesn't become active.
Using 1 or any other integer I'm able to debug the function.

I'm using
pgadmin Version 2.1
Python Version 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59) [MSC
v.1500 32 bit (Intel)]
Flask Version 0.12.2
Application Mode Desktop

on Win10 but this happens also on CentOS 7 with
pgadmin Version 2.1
Python Version 2.7.5 (default, Aug 4 2017, 00:39:18) [GCC 4.8.5 20150623
(Red Hat 4.8.5-16)]
Flask Version 0.11.1
Application Mode Desktop

Postgresql Server (10.2 + 9.6.7) runs on CentOS 7

sample function:

CREATE OR REPLACE FUNCTION public.test_dbg_integer(myint integer)
RETURNS text
LANGUAGE 'plpgsql'
VOLATILE STRICT
PARALLEL UNSAFE
COST 100
AS $BODY$
DECLARE outtxt text;--
BEGIN
outtxt = (myint)::text;--
RETURN outtxt;--
END;--
$BODY$;

kind regards
Georg