domains, case statements, functions: bug?

create domain m numeric(5,2);
create table t (c m);
create function f(t) returns m as $ select case when true then $1.c end $ language sql;


psql:/tmp/t1.sql:3: ERROR:  return type mismatch in function declared to return m
DETAIL:  Actual return type is numeric.
CONTEXT:  SQL function "f"