Need help with an if statement, C#

As the title implies, I'm writing a pretty basic program to explore some of what I know.

It's a simple morse translator in C#, my question arises around this last issue I have with it.  I'm in the process of writing the converter from Morse to English

http://pastebin.com/VUCwtF0j

Line 273 is where the error occurs.  C# Express is telling me I'm trying to implicitly convert from string to a bool, anyone mind helping me with a different way to do this?

That is actually a nice program, with some good ideas I actually never tried to use (specifically the Clipboard stuff).

Try to put yourself a challenge later, like making it in less than 300 lines, or doing a basic GUI for it.

I think the string concatenation you are doing is considered harmful, so get a basic grab of how the String class works and try to do the same thing using it instead of the "+=", for example. It'll be good to expand a little your knowledge of the language and the framework. Also, you'll find MUCH MORE ENTERPRISE verbose crap around in a few months, so try to not be afraid.