Fix no pk issue #2638

[ghost]

Updated patch addressing @thedrow 's comments.

[carltongibson]

I second @thedrow's comment on #2639 that there should be a test case here. It needs to demonstrate the issue that's being fixed. It should fail without the patch and pass with it.

Such tests bring to light the expected behaviour — they help to decide if that's the behaviour we want. (They also catch regressions down the line.)

My immediate thought though is that this looks similar to #2637 so we need to make sure we're not making two related changes where one is required. (That's another way the test would help.)

[carltongibson]

Closing as per comment on #2638