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.