PostgreSQL seems to ignore RAISE EXCEPTION in a CTE

Using PostgreSQL 9.3 I have been trying to define an assert helper function to check for empty query results and similar things as follows:

CREATE FUNCTION public.assert (    in_assertion boolean,    in_errormessage text)RETURNS booleanIMMUTABLELANGUAGE plpgsqlSECURITY INVOKERAS $function$    BEGIN        IF NOT in_assertion THEN            RAISE EXCEPTION 'assertion failed: %', in_errormessage;        END IF;        RETURN in_assertion;    END;$function$;

Upon testing I found that the exception is not thrown as I would expect. For example, with CREATE TABLE emptytable (somecolumn text); and

CREATE FUNCTION public.testassert_buggy (    out somevalue text)LANGUAGE sqlSECURITY DEFINERAS $function$    WITH firstquery AS (            SELECT * FROM emptytable    ), nonemptycheck AS (            SELECT assert(count(*) = 42, 'nonemptycheck failed') FROM firstquery    ) SELECT * FROM firstquery;$function$;

I would expect a call like SELECT testassert_buggy(); to throw the exception, but instead the result is

 somevalue -----------(1 row)

(Note that firstquery actually returns 0 rows; the 1 row is due to this being a function with out parameters.)

With the following small change in the second-to-last line, the exception IS thrown.

CREATE FUNCTION public.testassert (    out somevalue text)LANGUAGE sqlSECURITY DEFINERAS $function$    WITH firstquery AS (            SELECT * FROM emptytable    ), nonemptycheck AS (            SELECT assert(count(*) = 42, 'nonemptycheck failed') FROM firstquery    ) SELECT firstquery.* FROM nonemptycheck, firstquery;$function$;

If I rewrite the last query switching the table list (i.e. with FROM firstquery, nonemptycheck) there is again no exception. I'm puzzled. Is the query optimized in some way that ignores side-effects like exceptions? I tried to remove IMMUTABLE from the definition of assert, but that didn't make a difference.