Why can't I use weakly-named assembly in strongly-named one

I have simple .csproj which is signed with a key. When I installed package form nudget packet manager I got exception that: Cannot load file 'XXXX' or one of it's dependencies. Package with strong name is required (HRESULT: 0x80131044)

When I disabled signing my own .dll everything works. I am reading about strongly named assemblies and GAC but I don't see explanation why I can't use not signed package in signed one. Any idea? Am I missing something?