Indexing mappings with arrays fail

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1894

Reported by Marcus Agehall agehall@roxen.com

When creating a mapping m=([ ({1,1}):1 ]) I am unable to index it.
m[({1,1})] returns 0 while m[indices(m)[0]] returns ({1,1}).