Sometimes the order of the Interceptors makes a difference. In the case of the prepare Interceptor, ensure that it comes before validation on the interceptor-stack.

<interceptor-stack name="myInterceptorStack">
        ...
     <interceptor-ref name="prepare" />
        ...
     <interceptor-ref name="validation" />
        ...
  </interceptor-stack>

The default stack does place the prepare Interceptor before the validation Inteceptor.