Stack trace when attempting to add mirror disk

I attempted to attach da4 to da3 to create a mirror in testpool on this system. To do this, I went to Storage -> Pools -> gear menu for testpool -> status -> vertical menu for da3 -> extend, selected da4, and clicked Extend. Almost immediately, the system responded with a stack trace, the end of which was that the device was too small. There are two problems with this:

1. The system shouldn't give a stack trace on a perfectly ordinary error condition; it should give an intelligible error message--something like "ERROR: New device (XX GB) is smaller than existing device (XX GB)".

2. The new disk is exactly the same capacity as the existing one, down to the byte, so it shouldn't be too small.