[argyllcms] Dispcal - Inverting Jacobian failed?

Hi,
Quick question - what does the error message "Inverting Jacobian failed" from dispcal mean?
I have a dirt-cheap TFT monitor here whose response I expected to be pretty funky since there's no direct backlight brightness adjustment. Instead, the "brightness" control appears to adjust some form of gamma-like curve, and the "contrast" control... well I'm not sure exactly what it's doing yet - it looks like it could be scaling the range about a pivot point that depends on the the brightness control's setting!
Unsurprisingly it's a challenge to get this monitor operating at a bearable brightness without causing either clipping in the highlights or shadows.
Anyhow, my first attempts to calibrate it led to the above mentioned error being printed in the shell window - once at the beginning of each pass. What does the error mean?
Despite the error, the resulting calibration looks pretty good. The only quality issue I see, having performed calibration and profiling, is a visible transition between white and the lightest visible grey in a greyscale ramp. 

All the best,
--
Alastair M. Robinson

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