[Freesurfer] question about qdec

Hi all: I have three groups of subjects (major depression disorder,slight depression disorder and contrast),i want to use qdec to obtain the statistical significant areas containing difference of cortical thickness measurment among them. But there are some errors appearing when i run the group analysis:

ERROR: QdecGlmDesign::GenerateContrasts: factor 2 must have 2 levels

I think the error is related to the diagnosis.levels file which has 3 levels because i can run it successfully after removing any one of them.But i don't know why. Can you give me some help or suggestions? I have saved error information into the error.log file(see attachments)