Unexpected runtime error

The runtime has encountered an unexpected error.

Error source

Page Name:/QuotesBy.xsp
Control Id: _id138

Exception

Error while executing JavaScript computed expression
Script interpreter error, line=2, col=94: 'viewScope.firstNo' is null

JavaScript code

   1: 
   2: 								return "["+viewScope.firstNo.toString()+"-"+viewScope.lastNo.toString()+"] of "+viewScope.count.toString()+" "+viewScope.Author+" quotes";
   3: 							

Stack Trace