Google Web Toolkit - XSRF Protected Services : Invalid RPC Token - gwt

Question

I've implemented XSRF Protected Services in GWT project. I'm using GWT 2.6.0 release. When I try to load my app in a browser I get a very strange exception as follows:
Uncaught com.google.gwt.user.client.rpc.RpcTokenException: Invalid RPC token (Invalid RpcToken type: expected 'com.google.gwt.user.client.rpc.XsrfToken' but got 'class com.google.gwt.user.client.rpc.XsrfToken')
I've searched my classpath and I only have one XsrfToken class provided by gwt-servlet.jar located inside my WAR file. I downloaded 2.6 code from GIT and I see the code that is throwing the exception is provided by ProxyCreator.java in the method generateCheckRpcTokenTypeOverride.
Does anyone have any idea as to why this exception would be thrown. The error indicates to me at least that it should pass given that what is expected is what it has.
I'm pasting the method in for completeness:
protected void generateCheckRpcTokenTypeOverride(SourceWriter srcWriter, TypeOracle typeOracle,
SerializableTypeOracle typesSentFromBrowser) {
JClassType rpcTokenType = typeOracle.findType(RpcToken.class.getName());
JClassType[] rpcTokenSubtypes = rpcTokenType.getSubtypes();
String rpcTokenImplementation = "";
for (JClassType rpcTokenSubtype : rpcTokenSubtypes) {
if (typesSentFromBrowser.isSerializable(rpcTokenSubtype)) {
if (rpcTokenImplementation.length() > 0) {
// >1 implematation of RpcToken, bail
rpcTokenImplementation = "";
break;
} else {
rpcTokenImplementation = rpcTokenSubtype.getQualifiedSourceName();
}
}
}
if (rpcTokenImplementation.length() > 0) {
srcWriter.println("#Override");
srcWriter.println("protected void checkRpcTokenType(RpcToken token) {");
srcWriter.indent();
srcWriter.println("if (!(token instanceof " + rpcTokenImplementation + ")) {");
srcWriter.indent();
srcWriter.println("throw new RpcTokenException(\"Invalid RpcToken type: " + "expected '"
+ rpcTokenImplementation + "' but got '\" + " + "token.getClass() + \"'\");");
srcWriter.outdent();
srcWriter.println("}");
srcWriter.outdent();
srcWriter.println("}");
}
}
Thanks very much in advance.