The key don't necessary exists in the opposite object.

Update to revision https://github.com/camptocamp/mapfish-print/commit/86ac34ccb4bc3023dcc98620d09596e4125a853d
of https://github.com/camptocamp/mapfish-print/tree/c2cgeoportal-1.3
see also: https://github.com/mapfish/mapfish-print/pull/78