@@ -999,10 +999,16 @@
 	OFMutableString *new;
 
 	new = [OFMutableString stringWithString: self];
 	[new appendString: str];
 
+	/*
+	 * Class swizzle the string to be immutable. We declared the return type
+	 * to be OFString*, so it can't be modified anyway. But not swizzling it
+	 * would create a real copy each time -[copy] is called.
+	 */
+	new->isa = [OFString class];
 	return new;
 }
 
 - (BOOL)hasPrefix: (OFString*)prefix
 {
@@ -1054,10 +1060,16 @@
 	}
 	[array addObject: [OFString stringWithCString: string + last]];
 
 	[pool release];
 
+	/*
+	 * Class swizzle the array to be immutable. We declared the return type
+	 * to be OFArray*, so it can't be modified anyway. But not swizzling it
+	 * would create a real copy each time -[copy] is called.
+	 */
+	array->isa = [OFArray class];
 	return array;
 }
 
 - (intmax_t)decimalValue
 {