loading spritesheets in monogame extended

Can anyone help me with an exception that extended throws?
i told it to:

SpriteSheet spriteSheet = Content.Load<SpriteSheet>("Rat.sf", new JsonContentLoader());

and it told me

Microsoft.Xna.Framework.Content.ContentLoadException: 'Could not find ContentTypeReader Type. Please ensure the name of the Assembly that contains the Type matches the assembly in the full type name: MonoGame.Extended MonoGame.Extended.Animations.SpriteFactory.SpriteFactoryFileReader, MonoGame.Extended.Animations (MonoGame.Extended MonoGame.Extended.Animations.SpriteFactory.SpriteFactoryFileReader, MonoGame.Extended.Animations)'

I havn’t the faintest idea what this means and would like some help