Absolute expression for .if

I'm trying to add an assemble-time check on a section's size. The
basic approach is:

start_label:
...
end_label:

.ifgt (.-start_label)-256
.err
.endif

I'm using GNU assembler 2.14 20030612. The documentation for the -
operator says "If both arguments are in the same section, the result
is absolute." but the assembler says this is a non-constant
expression. (the documentation says .if requires an "absolute
expression")

I've tried a bunch of variations, but can't see to get passed the "non-
constant expression" error. Am I SOL here, or is there something else
I can do.

Thanks.