use a relational operator block along with an assertion block to check that your integrator input is always finite. If not so, try to trace it back and fix it there to always be finite.
If the input is finite, there is the possibility you ran into a simulink bug that causes a completely wrong error message. I had a table lookup where the lookup data contained a nan-value. Whenever it had to use this value for interpolation, i got above error message for an integrator far away from the table lookup, even when i made sure a nan-output from the lut is never used by passing the output through a switch controlled by an isnan relational operator, replacing nan by a 1 in case. i corrected the lut data and the problem was solved.