While using the WcfTestClient, I received the error message displayed in the title. It turned out that in my case, I was using colons in the namespaces of my wcf service, like so:
[OperationContract(Action = “urn:SomeName::SomeMoreNames”)]
I managed to fix the error by changing the namespaces into something like “http://SomeName.SomeMoreNames”. Hope this helps anyone.