Problem with FillSinks in Python script

We're running a number of preprocessing tools in ArcHydro, including FillSinks. All of the other tools we are using work, but when the python script calls FillSinks:

# Process: Fill Sinks
ArcHydroTools.FillSinks(AgreeDEM, Fil, "", LevelPoly, "ISSINK_YES")

we get this error message:

Traceback (most recent call last):
File "U:\ArcHydro\ArcHydroPhilip\AutoHydro_beta2.py", line 101, in <module>
ArcHydroTools.FillSinks(AgreeDEM, Fil, "", LevelPoly, "ISSINK_YES")
File "C:\Program Files\ESRI\WaterUtils\ArcHydro\bin\ArcHydroTools.py", line 2319, in FillSinks
raise e
ExecuteError: System.NullReferenceException: Object reference not set to an instance of an object.

at ESRI.APWR.ArcHydroGPTools.AHFillSinks.Execute(IArray paramvalues, ITrackCancel trackcancel, IGPEnvironmentManager envMgr, IGPMessages message) in C:\Builds\HydroSolutions\10.0_ArcHydro\Sources\ArcHydroTools\src\ArcHydroGPTools\AHFillSinks.vb:line 398
Failed to execute (FillSinks).

We're not sure why this is occurring, but any help is appreciated! Thanks!