Sorry for not replying to this sooner - this was actually fixed a few revisions ago. Please find the latest version of the code <a href="https://github.com/astrofrog/idlsave">here</a>.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
Complex numbers were implemented in IDLSave 0.9.7. Could you try downloading this version, and seeing if you still get the error?
Sorry for not replying to this sooner - this was actually fixed a few revisions ago. Please find the latest version of the code <a href="https://github.com/astrofrog/idlsave">here</a>.