The method call requires an assembly reference. What for?

Assembly A

class A {
    void SaveToDisk(); // Calls B.Extract
}

      

        
        
        
      

    

Assembly B

class B {
    static void Extract(); // References Ionic.Zip
}

      

        
        
        
      

    

The compiler tells me that Assembly A requires a reference to Ionic.Zip, although Assembly A does not have any references to any types in Ionic.Zip in the SaveToDisk method. Why does assembly A require an ionic assembly reference?