Help! mach-O linker error


I have had no compiler issues until now. beginning chapter 22 with the first lines of code:

    boxLayer = [[CALayer alloc] init];

leads to two errors, the first of which is:

Apple Mach-O linker (id) error “objc_class$_calayer”, …

description includes

OBJC_CLASS$_CALayer”, referenced from:
Objc-class-ref in HypnosisView.o
Symbol(s) not found for architecture i386

I can’t find anything on how to resolve this.