Ccontent-type is null with Google Structured Data testing tool?

When accessing my GWT website with Google Structured Data testing tool, I get the following error:

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javax.servlet.ServletException: Content-Type was '(null)'. Expected 'text/x-gwt-rpc'.      at com.google.gwt.user.server.rpc.RPCServletUtils.checkContentTypeIgnoreCase(RPCServletUtils.java:477)      at com.google.gwt.user.server.rpc.RPCServletUtils.readContent(RPCServletUtils.java:209)

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The detected user agent:

[mozilla/5.0 (compatible; google-structured-data-testing-tool +http://developers.google.com/structured-data/testing-tool/)] How must I solve/deal with this ? - Ed

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