pipeline extension reference not working

i want to write a custom importer but i can’t get the reference to LevelLibrary to work.
it works when Level.cs is inside of ContentPipelineExtension.

in Content.mgcb
-------------------------------- References --------------------------------
/reference:…\ContentPipelineExtension\bin\Debug\LevelLibrary.dll
/reference:…\ContentPipelineExtension\bin\Debug\ContentPipelineExtension.dll

in LevelProcessor
using TOutput = LevelLibrary.Level;

Why?