invertIon.py Error: dates not connected through least squares

[BrieCorsa]

Hello all!
I am using the ISCE2 Stack Processor (topsStack) to develop my interferograms with the ionospheric correction. I am getting the following error from the final step: invertIon.py (line 259): Exception: dates to be estimated are not fully connected by the pairs used in least squares.

Here is a screenshot of the code in question:

On lines 249-250, it says, "if rank < ndate-1: raise Exception('dates to be estimated are not fully connected by the pairs used in least squares')"
My ndate is a size of 56, my H0 matrix is (156, 55), and my rank comes out to 54- so because my rank is 2 less than ndate (as opposed to 1 less than ndate), it kicks me out of the processing routine with this error. Can anyone tell me why this is happening? (The number of possible dates is 56 and the number of used dates is also 56, so I am stuck on why my rank is 54 instead of 55?)