Plugin on Quote Win InputParameters.Contains("Target") issue

I have a plugin that runs on Quote Win. However there seems to be some issue with getting the inputparaments. I'm assuming because the quote is active and read only at this point that I must use the pre-entity image values? However I've tried a couple different code snippets and both thrown and exception.

if (context.InputParameters.Contains("Target") &&
           context.InputParameters["Target"] is Entity)

if (context.PreEntityImages.Contains("Target") &&
               context.PreEntityImages["Target"] is Entity)

Neither bit works. I have an else statement that throws an exception. Any ideas what I'm doing wrong? This works fine if the plugin runs on Quote Update. Just not Quote Win...

Thanks for any help.