Line_exists - odd compiler error

I was trying to use line_exists today and ran into an unexpected issue you can see below. When I use line_exists for an if statement expression, it handles the usage correctly. If I try to save the value directly to a boolean variable however, it will fail with the odd "line_exists( expected, not line_exists(" error. Just curious if anybody knew if there was a note out there to address this problem or if this is not allowed usage for line_exists.