Yes, @timo is correct, support for ic0.is_controller was only added in moc v0.9.1.
@qwertytrewq you would need at least version 0.9.1 of both the compiler and the base package.
The DFX_MOC_PATH env variable allows you to do this, so you do not need to wait on a DFX release.
DFX v0.14.0 only comes with Motoko compiler 0.8.7.