Simple expression to add value to array

Summary

Simple expression to add value to array

Content

I have an array body.creditLine.  I need to add a value to the end of that array.  I use the following:

body.creditLine[body.creditLine.length].lineAmount

this works and validates fine, however after i apply it then go back in it switches to

body.creditLine[body.creditLine.length()].lineAmount and shows as invalid.  However it still passes validation fine, but then it doesnt work when it runs.  it doesnt fail, just doesnt add the value