Solving in C# - fairly confident I have a somewhat optimal solution. When I run tests I get the following output:

Test Results:
  TotalIncDecTests
  BasicTests (1 Passed, 0 Failed)
You have passed all tests! :)

However, when I click submit I get the following error:

Test Results:
  TotalIncDecTests
  BasicTests
 System.ArgumentNullException : Value cannot be null.
Parameter name: destinationArray
 Completed in 2.431534ms
  RandomTests
 System.ArgumentNullException : Value cannot be null.
Parameter name: destinationArray
 Completed in 2.121ms

No idea what that's about or where this destinationArray parameter is coming from. Why would BasicTests error out when I submit but pass when I run tests?