Partitioning / constrain exlusion not working with %-operator

I try to partition a large table (~ 120 mio. rows) into 50 smaller
tables but using the IMO immutable %-function constraint exclusion
does not work as expected:

CREATE TABLE tt_m (id1 int, cont varchar);
CREATE TABLE tt_0 (check (id1 % 50 = 0)) INHERITS (tt_m);
CREATE TABLE tt_1 (check (id1 % 50 = 1)) INHERITS (tt_m);
....
CREATE RULE ins_tt_0 AS ON INSERT TO tt_m WHERE id1 % 50 = 0 DO INSTEAD INSERT INTO tt_0 VALUES (new.*);
CREATE RULE ins_tt_1 AS ON INSERT TO tt_m WHERE id1 % 50 = 1 DO INSTEAD INSERT INTO tt_1 VALUES (new.*);
...
INSERT INTO tt_m (id1,cont) VALUES (0,'Test1');
INSERT INTO tt_m (id1,cont) VALUES (1,'Test2');
....
EXPLAIN SELECT * FROM tt_m WHERE id1=1;
QUERY PLAN
-----------------------------------------------------------------------
Result (cost=0.00..73.50 rows=18 width=36)
-> Append (cost=0.00..73.50 rows=18 width=36)
-> Seq Scan on tt_m (cost=0.00..24.50 rows=6 width=36)
Filter: (id1 = 1)
-> Seq Scan on tt_0 tt_m (cost=0.00..24.50 rows=6 width=36)
Filter: (id1 = 1)
-> Seq Scan on tt_1 tt_m (cost=0.00..24.50 rows=6 width=36)
Filter: (id1 = 1)
...

Only adding an explicit %-call to the query results in the expected plan:

EXPLAIN SELECT * FROM tt_m WHERE id1=1 AND id1 % 50 = 1;
QUERY PLAN
-----------------------------------------------------------------------
Result (cost=0.00..60.60 rows=2 width=36)
-> Append (cost=0.00..60.60 rows=2 width=36)
-> Seq Scan on tt_m (cost=0.00..30.30 rows=1 width=36)
Filter: ((id1 = 1) AND ((id1 % 50) = 1))
-> Seq Scan on tt_1 tt_m (cost=0.00..30.30 rows=1 width=36)
Filter: ((id1 = 1) AND ((id1 % 50) = 1))

Did I miss something and/or how could I force the planner to use
constraint exclusion without adding the explicit second condition above?

TIA, Martin