Index: src/OFDictionary.h
==================================================================
--- src/OFDictionary.h
+++ src/OFDictionary.h
@@ -210,10 +210,25 @@
  * @param key The key whose value should be returned
  * @return The value for the given key or `nil` if the key was not found
  */
 - (nullable id)valueForKey: (OFString*)key;
 
+/*!
+ * @brief Sets a value for a key.
+ *
+ * If the key starts with an `@`, the `@` is stripped and
+ * `[super setValue:forKey:]` is called.
+ * If the key does not start with an `@`, this is equivalent to
+ * @ref setObject:forKey:. In this case, if the dictionary is immutable, an
+ * @ref OFUndefinedKeyException is thrown.
+ *
+ * @param key The key to set
+ * @param value The value to set the key to
+ */
+- (void)setValue: (nullable id)value
+	  forKey: (OFString*)key;
+
 /*!
  * @brief Checks whether the dictionary contains an object equal to the
  *	  specified object.
  *
  * @param object The object which is checked for being in the dictionary

Index: src/OFDictionary.m
==================================================================
--- src/OFDictionary.m
+++ src/OFDictionary.m
@@ -27,10 +27,11 @@
 #import "OFXMLElement.h"
 #import "OFDataArray.h"
 
 #import "OFInvalidArgumentException.h"
 #import "OFOutOfRangeException.h"
+#import "OFUndefinedKeyException.h"
 
 static struct {
 	Class isa;
 } placeholder;
 
@@ -296,10 +297,33 @@
 		return [ret autorelease];
 	}
 
 	return [self objectForKey: key];
 }
+
+- (void)setValue: (id)value
+	  forKey: (OFString*)key
+{
+	if ([key hasPrefix: @"@"]) {
+		void *pool = objc_autoreleasePoolPush();
+
+		key = [key substringWithRange: of_range(1, [key length] - 1)];
+		[super setValue: value
+			 forKey: key];
+
+		objc_autoreleasePoolPop(pool);
+		return;
+	}
+
+	if (![self isKindOfClass: [OFMutableDictionary class]])
+		@throw [OFUndefinedKeyException exceptionWithObject: self
+								key: key
+							      value: value];
+
+	[(OFMutableDictionary*)self setObject: value
+				       forKey: key];
+}
 
 - (size_t)count
 {
 	OF_UNRECOGNIZED_SELECTOR
 }

Index: src/OFMutableDictionary.h
==================================================================
--- src/OFMutableDictionary.h
+++ src/OFMutableDictionary.h
@@ -75,24 +75,10 @@
 - (void)setObject: (ObjectType)object
 	   forKey: (KeyType)key;
 -   (void)setObject: (ObjectType)object
   forKeyedSubscript: (KeyType)key;
 
-/*!
- * @brief Sets a value for a key.
- *
- * If the key starts with an `@`, the `@` is stripped and
- * `[super setValue:forKey:]` is called.
- * If the key does not start with an `@`, this is equivalent to
- * @ref setObject:forKey:.
- *
- * @param key The key to set
- * @param value The value to set the key to
- */
-- (void)setValue: (nullable id)value
-	  forKey: (OFString*)key;
-
 /*!
  * @brief Removes the object for the specified key from the dictionary.
  *
  * @param key The key whose object should be removed
  */

Index: src/OFMutableDictionary.m
==================================================================
--- src/OFMutableDictionary.m
+++ src/OFMutableDictionary.m
@@ -175,28 +175,10 @@
 {
 	[self setObject: object
 		 forKey: key];
 }
 
-- (void)setValue: (id)value
-	  forKey: (OFString*)key
-{
-	if ([key hasPrefix: @"@"]) {
-		void *pool = objc_autoreleasePoolPush();
-
-		key = [key substringWithRange: of_range(1, [key length] - 1)];
-		[super setValue: value
-			 forKey: key];
-
-		objc_autoreleasePoolPop(pool);
-		return;
-	}
-
-	[self setObject: value
-		 forKey: key];
-}
-
 - (void)removeObjectForKey: (id)key
 {
 	OF_UNRECOGNIZED_SELECTOR
 }
 

Index: tests/OFDictionaryTests.m
==================================================================
--- tests/OFDictionaryTests.m
+++ tests/OFDictionaryTests.m
@@ -21,10 +21,11 @@
 #import "OFArray.h"
 #import "OFNumber.h"
 #import "OFAutoreleasePool.h"
 
 #import "OFEnumerationMutationException.h"
+#import "OFUndefinedKeyException.h"
 
 #import "TestsAppDelegate.h"
 
 static OFString *module = @"OFDictionary";
 static OFString *keys[] = {
@@ -57,10 +58,14 @@
 
 	TEST(@"-[valueForKey:]",
 	    [[dict valueForKey: keys[0]] isEqual: values[0]] &&
 	    [[dict valueForKey: @"@count"] isEqual:
 	    [OFNumber numberWithSize: 2]])
+
+	EXPECT_EXCEPTION(@"Catching -[setValue:forKey:] on immutable "
+	    @"dictionary", OFUndefinedKeyException, [dict setValue: @"x"
+							    forKey: @"x"])
 
 	TEST(@"-[containsObject:]",
 	    [dict containsObject: values[0]] &&
 	    ![dict containsObject: @"nonexistant"])