undefined reference to `ctl_unmask_isr'

I've written around a zillion different ARM projects and have just started a new one, got some stuff working then imported a uart.c file I wrote and used several times before. I am now getting thie following error:

undefined reference to `ctl_unmask_isr'

I have all the same includes as with a project that does work and am baffled as to why this error is coming up. If I go back to another project it's fine.

Anyone able to shed some light please? Thanks.