diff --git a/src/pymor/grids/boundaryinfos.py b/src/pymor/grids/boundaryinfos.py
index a291dcb9da..47fb52cef1 100644
--- a/src/pymor/grids/boundaryinfos.py
+++ b/src/pymor/grids/boundaryinfos.py
@@ -39,7 +39,7 @@ def __init__(self, grid, indicators, assert_unique_type=None, assert_some_type=N
         self.grid = grid
         assert_unique_type = assert_unique_type if assert_unique_type else [1]
         assert_some_type = assert_some_type if assert_some_type else []
-        self.boundary_types = indicators.keys()
+        self.boundary_types = frozenset(indicators.keys())
         self._masks = {boundary_type: [np.zeros(grid.size(codim), dtype='bool') for codim in xrange(1, grid.dim + 1)]
                        for boundary_type in self.boundary_types}
         for boundary_type, codims in self._masks.iteritems():