src/de/steamwar/command/SWCommandUtils.java

@@ -46,7 +46,7 @@ public class SWCommandUtils {
 
         @Override
         public Collection<String> tabCompletes(Object sender, String[] previousArguments, String s) {
-            return tabCompleter.apply(sender, s);
+            return ((BiFunction<Object, Object, Collection<String>>) tabCompleter).apply(sender, s);
         }
     };