Undefined function 'minus' for input arguments of type 'struct'

I am trying to use the ANFIS GUI for fuzzy network processing of some data (files available upon request). I have gone through the initial phases of loading training and checking data sets, generating an automatic FIS, and running the training algorithm based on the default parameters suggested in the Gershteyn and Perman (2003) tutorial. However, when I come to run the Test FIS using the checking data, I get the message

"Undefined function 'minus' for input arguments of type 'struct'"

This error is obtained using the example data sets fuzex1trnData and fuzex1chkData, as well as using my own data sets generated from tool data tests.

Does anyone know how to cure this error?

Thanks Noel