Membership - error when entering contribution

Folks,

I have set up a membership organisation within which, each membership level has a level code that is unique.  The membership level values are set with start and end amounts which neatly dovetail but don't overlap.

My issue is that when trying to make a Contribution that should trigger one of these membership levels I receive the error message:

"This amount returns no level and there is no current membership to upgrade"

Sadly, this then generates an ugly error that forces the client to close or be restarted.

Can anyone shed any light on what might be missing from my setup?

Thanks!