I'm modeling a combination of steel wire ropes (nonlinear elastic) and polyurethane (hyperelastic Mooney Rivlin), but I keep getting an error that I can't sort out. The error message is "Error in MPC input: Unable to find tangent modulus from the curve because strain(i) is equal to strain(i+1)"

Because it mentions strain instead of stretch ratio, I'm thinking the problem is in the wire rope material definition.  I've modeled the ropes as simple cylinders of an equivalent area with nonlinear elastic properties defined by a stress strain curve from test data.  I've checked, double-checked, and triple checked, but both the stress and strain elements of the curve have no duplicates and are monotonically increasing so I can't figure out why I'm getting this error. Does anyone have any insight to what I'm doing wrong?  I've attached an image of the stress/strain curve.

While I think it's the rope definition, I've also checked my hyperelastic material curve for the polyurethane, and it's also monotonically increasing with no duplicates.

I also find that after a failed run, when I go to look at the material properties, they have reverted to linear elastic isotropic.  I've checked that they do save properly before I run the simulation.