First, this doesn't sound Seam related. Go ask on the forums for the gwt-maps library.
Secondly, if you actually want help with a problem then think about how you ask for help. You need to give as much information as possible. Library versions, Source code for the entire call not just 1 line. Was this code inside a Seam bean? If so what scope? etc
Third, if you get an exception you MUST ALWAYS provide the ENTIRE stacktrace. Providing just the name of the exception is like me asking you to add two numbers and give me the answer but I wont tell you what the numbers are.
So, get the full stacktrace. Take the time to see if it has anything to do with Seam. If yes then post the stacktrace here. If not, go ask for help from the people who wrote the library your using. But, before you do that go read the doco. Prove to the people you are asking for help from that you have actually tried to solve the problem yourself, not just getting others to do your work for you.