Can't create minimap in World Partition

Hello,
as I try to build a minimap for a simple test-level I get this error in the log:

LogWindows: Error: appError called: Assertion failed: InActor->IsPackageExternal() [File:D:\Unreal\UnrealEngine-5.1\Engine\Source\Runtime\Engine\Private\WorldPartition\WorldPartitionActorDesc.cpp] [Line: 43]

Curently I using ue 5.1, is there a known issue arround building minimaps?