When we pass some special words, NSString.getBytes does not crash even when we pass an invalid range. It seems a bug. The below code is an example. func testNSStringGetBytes() { let originalString: String = "􁜁あ" let bufferSize = 256 var buffer = [UInt8](repeating: 0, count: bufferSize) var usedLength = 0 // An invalid range is passed let range = NSRange(location: 0, length: originalString.count + 1) var remainingRange = NSRange() (originalString as NSString) .getBytes( &buffer, maxLength: bufferSize, usedLength: &usedLength, encoding: String.Encoding.utf8.rawValue, options: [], range: range, remaining: &remainingRange ) print("Used Length: \(usedLength)") print("Buffer: \(buffer[0..<usedLength])") if remainingRange.length > 0 { print("Did not convert the whole string. Remaining range: \(remainingRange)") } else { print("Entire string was converted successfully.") } }