Issue with substr function in v3.2

Dears,

In substr function length +1 value is giving worng result in the v3.2.

Please correct me, where I am doing wrong.

EX:

$x is integer

$x = substr( '12345',6,1);

print('Value is :' || $x );

Result : 0         (but it should give 'null')

kr

Narendra.