Edited Issue: Custom FxCop rule MW1200 crashes with NullReferenceException [1416]

This rule has been failing sporadically for some time. It recently started failing all the time in portable libraries.

CA0001 : Rule=Microsoft.Web.FxCop#MW1200, Target=System.Web.Http.Error.#Argument(System.String,System.Object[]) : Object reference not set to an instance of an object.

Code Analysis detected errors. See Code Analysis results window or log file for details.

Project Runtime\src\System.Net.Http.Formatting.NetCore\System.Net.Http.Formatting.NetCore.csproj failed.

Project Runtime\test\System.Net.Http.Formatting.NetCore.Test\System.Net.Http.Formatting.NetCore.Test.csproj failed.

Project Runtime\RuntimePortable.sln failed.