Index: src/OFSet.m
==================================================================
--- src/OFSet.m
+++ src/OFSet.m
@@ -253,11 +253,44 @@
 
 - (int)countByEnumeratingWithState: (of_fast_enumeration_state_t *)state
 			   objects: (id *)objects
 			     count: (int)count
 {
-	OF_UNRECOGNIZED_SELECTOR
+	OFEnumerator *enumerator;
+	int i;
+
+	memcpy(&enumerator, state->extra, sizeof(enumerator));
+
+	if (enumerator == nil) {
+		void *pool = objc_autoreleasePoolPush();
+
+		enumerator = [[self objectEnumerator] retain];
+		memcpy(state->extra, &enumerator, sizeof(enumerator));
+
+		objc_autoreleasePoolPop(pool);
+	}
+
+	state->itemsPtr = objects;
+	state->mutationsPtr = (unsigned long *)self;
+
+	if (state->state == 1)
+		return 0;
+
+	for (i = 0; i < count; i++) {
+		id object = [enumerator nextObject];
+
+		if (object == nil) {
+			state->state = 1;
+			[enumerator release];
+
+			return i;
+		}
+
+		objects[i] = object;
+	}
+
+	return i;
 }
 
 - (bool)isEqual: (id)object
 {
 	OFSet *set;

Index: tests/OFSetTests.m
==================================================================
--- tests/OFSetTests.m
+++ tests/OFSetTests.m
@@ -32,10 +32,11 @@
 @end
 
 @interface SimpleMutableSet: OFMutableSet
 {
 	OFMutableSet *_set;
+	unsigned long _mutations;
 }
 @end
 
 @implementation SimpleSet
 - (instancetype)init
@@ -115,19 +116,10 @@
 
 - (OFEnumerator *)objectEnumerator
 {
 	return [_set objectEnumerator];
 }
-
-- (int)countByEnumeratingWithState: (of_fast_enumeration_state_t *)state
-			   objects: (id *)objects
-			     count: (int)count
-{
-	return [_set countByEnumeratingWithState: state
-					 objects: objects
-					   count: count];
-}
 @end
 
 @implementation SimpleMutableSet
 + (void)initialize
 {
@@ -135,16 +127,39 @@
 		[self inheritMethodsFromClass: [SimpleSet class]];
 }
 
 - (void)addObject: (id)object
 {
+	bool existed = [self containsObject: object];
+
 	[_set addObject: object];
+
+	if (existed)
+		_mutations++;
 }
 
 - (void)removeObject: (id)object
 {
+	bool existed = [self containsObject: object];
+
 	[_set removeObject: object];
+
+	if (existed)
+		_mutations++;
+}
+
+- (int)countByEnumeratingWithState: (of_fast_enumeration_state_t *)state
+			   objects: (id *)objects
+			     count: (int)count
+{
+	int ret = [_set countByEnumeratingWithState: state
+					    objects: objects
+					      count: count];
+
+	state->mutationsPtr = &_mutations;
+
+	return ret;
 }
 @end
 
 @implementation TestsAppDelegate (OFSetTests)
 - (void)setTestsWithClass: (Class)setClass