+ # If the subalgebra is trivial, its _matrix_span will be empty
+ # but we still want to be able convert the superalgebra's zero()
+ # element into the subalgebra's zero() element. There's no great
+ # workaround for this because sage checks that your basis is
+ # linearly-independent everywhere, so we can't just give it a
+ # basis consisting of the zero element.
+ m = elt.to_matrix()
+ if self.is_trivial() and m.is_zero():
+ return self.zero()
+ else:
+ return super()._element_constructor_(m)