Invalid operands for binary and

Invalid operands for binary and

I have this "assembly" file (containing only directives)S
// declare protected region as somewhere within the stack
.equiv prot_start, $stack_top & 0xFFFFFF00 - 0x1400
.equiv prot_end, $stack_top & 0xFFFFFF00 - 0x0C00
Combined with this linker script:
SECTIONS {
"$stack_top" = 0x10000;
}
Assembling produces this output
file.s: Assembler messages:
file.s: Error: invalid operands (*UND* and *ABS* sections) for `&' when
setting `prot_start'
file.s: Error: invalid operands (*UND* and *ABS* sections) for `&' when
setting `prot_end'
How can I make this work?