From where in your code are you calling NavigateURL? Your module control should inherit from DotNetNuke.Entities.Modules.PortalModuleBase (directly or indirectly) which should insure that its property ModuleId returns a valid value. If, however, you are calling NavigateURL from a user control that has been placed as a child control of your module control you will have to either pass ModuleId into the child user control or as is often done, pass the module control's ModuleConfiguration (of type ModuleInfo) or the ModuleContext (of type ModuleInstanceContext) into the child user control.

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner