“) Syntax error Invalid arithmetic operator (error token is ”

Question

I have gone through all the possible similar questions on SO already & couldn't figure out the error.

echo $select_app
echo $bitmap
app_itx='echo $(($select_app & $bitmap))'

This script worked on machine with bash version 4.1.10(4)-release (i686-pc-cygwin).

but doesn't work on machine with version 4.4.12(3)-release (x86_64-unknown-cygwin).

Both machines are 64-bit running windows 7 & windows 10 respectively.

output of script :

0x04

1

& 1“) Syntax error Invalid arithmetic operator (error token is ”