Warning "numerical instability" when using PCKriging

Hi there,
Does anybody have this warning when using PCKriging metamodel?
numerical instability!! Gamma for LAR iteration 44 was set to 0 to prevent crashes.
It seems whenever I increase the number of input data ‘PCEKrigOpts.ExpDesign.X’, this will happen. For example, I have 100 input data X(2,100) and 100 output data Y(1,100), if I set “PCEKrigOpts.ExpDesign.X = X(2,1:20)” & “PCEKrigOpts.ExpDesign.Y = Y(1,1:20)”, there will be no Warning like this, but if I use “PCEKrigOpts.ExpDesign.X = X(2,1:100)” & “PCEKrigOpts.ExpDesign.Y = Y(1,1:100)”, it’ll give me the warning.
Anybody knows why will it happen?
Thanks a lot!
Best regards,