Parameter 'ad' not found in transistor models

I was trying to perform some cell characterization in liberate and I encountered this error:

*Error* (char_library) : Failed to find parameter : ad
ERROR (LIB-507): (char_library): Failed to process the model based element inst: 'PM0', model: 'MOS' in the cell Inv. This cell will be skipped. Check the model uses standard parameter names and the define_leafcell command options are used properly.  Update the Tcl script and rerun.

I suspected this to be the issue with the wrong corner being selected in the transistor model file. I rechecked the section   to be 'lp_rvt12'. What could  be going wrong here?

Thanks in advance!