Access Violation Error when creating an std::string from a unique_ptr char buffer

I'm currently trying to read a null-terminated ASCII string from another process. To do so, I created a class which acts as a memory reader with a generic readBytes function which acts as a wrapper for the Windows function ReadProcessMemory with error handling. My readBytes function works just fine, however my readAsciiString function (code below) is throwing Access Violation Errors when creating a new std::string from a unique_ptr char array.

std::string ProcessReader::readAsciiString(DWORD_PTR offset, int maxChars)
{
    auto buffer = std::make_unique<char[]>(maxChars);

    if (readBytes(offset, buffer.get(), maxChars))
    {
        int pos = 0;
        for (; pos < maxChars; pos++)
        {
            if (buffer[pos] == '�')
                break;
        }

        auto returnBuffer = std::make_unique<char[]>(pos);
        memcpy(returnBuffer.get(), buffer.get(), pos);

        // The program breaks here reporting an Access Violation
        // trying to writing to offset 0x00000000FFFFFFFF
        return std::string(returnBuffer.get());
    }

    return std::string("");
}

Here is a screenshot of the code in Visual Studio with a breakpoint on the return line while watching both buffer and returnBuffer. Resuming the program causes it to crash on that very line.

I'm relatively new to C++ and am struggling to see what I'm doing wrong. Am I approaching this the wrong way?