This bug was introduced sometime after v0.11.0 and has turned up in my test suite using v0.11.2. I'll pass a DataArray() as my concat_dim, and the failure will happen at line 609 in backends/api.py because of: if concat_dim is None or concat_dim == _CONCAT_DIM_DEFAULT:

I am not sure how this change got through. In #2048, I added a unit test that passes a DataArray as a concat_dim argument.