[ COMPILER MESSAGE ] : Error: Infer(unforced) kept changing stuff

I'm trying to compile a watch face that includes the following code to change the background colour as a simple low battery warning:

g.setBgColor((bngl && E.getBattery()<10)?63488:(g.theme.bg))

When compiled this gives:

WARNING: Function marked with "compiled" uploaded in source form
[ COMPILER MESSAGE ] : Error: Infer(unforced) kept changing stuff

If the line is commented out the warning doesn't appear and the compiled version runs.

What causes the error and is there a way around it? Thanks in advance.