Index: src/OFMutableSet.m
==================================================================
--- src/OFMutableSet.m
+++ src/OFMutableSet.m
@@ -141,10 +141,15 @@
 
 - initWithCapacity: (size_t)capacity
 {
 	OF_INVALID_INIT_METHOD
 }
+
+- copy
+{
+	return [[OFSet alloc] initWithSet: self];
+}
 
 - (void)addObject: (id)object
 {
 	OF_UNRECOGNIZED_SELECTOR
 }