AssistEdge #EM18 Error while type change

I am using AssistEdge 18.2 and get the following error when calling a microbot:

“level”:“Error”,“priority”:“0”,“srcmodule”:“EV.AE.Studio.ActivityLibrary.Utils.DataParser”,“method”:“ParseAttributeValue”,“errormessage”:“#EM18 Error while type change”,“errordetails”:"ExceptionMessage - Null object cannot be converted to a value type. \r\nExceptionSource - mscorlib \r\nExceptionTarget - System.Object ChangeType(System.Object, System.Type, System.IFormatProvider)

I printed out the input parameters before calling the microbot and nothing is null. I also checked that the parameters are declared as input parameters in the microbot.

The process and the microbot have not been changed and ran without error before. The version of AssistEdge did not change either. Do you know what this error/exception is telling me? How do I fix this? All the input parameters are of type String.

Thank you.