BUG: ddt FO throws warning message for legitimate input fields

Summary

For a volVectorField input, say U, ddt FO throws a warning message:

--> FOAM Warning :     functionObjects::ddt ddt1 cannot find required object U of type volScalarField

however in the subsequent line of log, reports the successful execution:

functionObjects::ddt ddt1 writing field: ddtU

Steps to reproduce

cd $FOAM_TUTORIALS/incompressible/pisoFoam/RAS/cavity

Append the following to controlDict:

functions
{
  ddt1
  {
    // Mandatory entries
    type            ddt;
    libs            (fieldFunctionObjects);
    field           U;

    // Optional (inherited) entries
    result          ddtU;
  }
}

Environment information

• OpenFOAM version : dev
• Operating system : osuse
• Compiler : clang7 && gcc4

Possible fixes

Another layer of type selection into ddt:calc() should solve the problem.