Issue with Simulink Update diagram

While the simulation is running, I changed a variable in the command window and tried to update diagram. However, when this happens Matlab opens a system error window saying:"Matlab has encountered an internal problem and needs to close". The details say that:"This error was detected while a MEX-file was running. If the MEX-file is not an official MathWorks function, please examine its source code for errors. Please consult the External Interfaces Guide for information on debugging MEX-files." What's going on?