@@ -1287,10 +1287,27 @@
 {
 	OFMutableString *new;
 
 	new = [OFMutableString stringWithString: self];
 	[new appendString: string];
+
+	[new makeImmutable];
+
+	return new;
+}
+
+- (OFString*)stringByAppendingFormat: (OFConstantString*)format, ...
+{
+	OFMutableString *new;
+	va_list arguments;
+
+	new = [OFMutableString stringWithString: self];
+
+	va_start(arguments, format);
+	[new appendFormat: format
+		arguments: arguments];
+	va_end(arguments);
 
 	[new makeImmutable];
 
 	return new;
 }