Path 'POST' is forbidden

[Guest]

I am trying to test a simple Web Service and when I invoke a Web Method
that returns a Dataset I get:

Path 'POST' is forbidden.

Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information
about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Path 'POST' is forbidden.

Source Error:

An unhandled exception was generated during the execution of the current web
request. Information regarding the origin and location of the exception
can be identified using the exception stack trace below.

Stack Trace:

[HttpException (0x80004005): Path 'POST' is forbidden.]
System.Web.HttpMethodNotAllowedHandler.ProcessRequest(HttpContext
context) +79

System.Web.CallHandlerExecutionStep.System.Web.HttpApplication+IExecutionStep.Execute()
+179 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously) +87

Any ideas please?