C# calling C++ 3rd party DLL (no source) raises exception - not PInvoke Compatible

I am using VS 2015 Community with an ASP.NET MVC Web Application that uses a 3rd party C++ DLL I do not have source code for. Documentation is very scarce as is any helpful communication with the authors of the 3rd party DLL.

I've asked a related SO Question and received a good answer from @Steven. I've modified my code according to his answer and am trying to make a successful call to the 3rd party C++ DLL. The code:

// Call DLL
MyDLLInput _DLLInput = new MyDLLInput();
{
    SomeList = new int[288],
    ...
    SomeInt = 22,
    SomeDbl = 1.45,
    ...
    PathtoData = new byte[256]
};    

var ids = new int[] { 0, 12, 33, 67, 93 };
Array.Copy(ids, _DLLInput.SomeList, ids.Length);

var pathToDataBytes = Encoding.UTF8.GetBytes("C:\Some\Path\To\Data");
Array.Copy(pathToDataBytes, _DLLInput.PathtoData, pathToDataBytes.Length);

// Call DLL Entry Point

MyDLLOutput _DLLOutput = MyDLL.Unit(_DLLInput);

Raises exception:

Method's type signature is not PInvoke compatible.

The input/output structs and DLLImport() declarations:

// Input STRUCT

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct MyDLLInput
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 288)]
    public int[] SomeList;
    ...
    public int SomeInt;
    public int SomeDbl;
    ...
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 256)]
    public byte[] PathtoData;
};

// Output STRUCT

[StructLayout(LayoutKind.Sequential)]
public struct MyDLLOutput
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 288)]
    public int[] SomeList;
    ...
    public int SomeInt;        // Same as input
    public double SomeDbl;     // Same as input
    ...
}

// DLL Entry Point

public class MyDLL
{
    [DllImport("My_DLL.dll",
        EntryPoint = "?Unit@@YA?AUDLLOutput@@UDLLInput@@@Z",
        CallingConvention = CallingConvention.Cdecl)]
    public static extern MyDLLOutput Unit(MyDLLInput UnitInput);
}

I think I must be close, but haven't been able to find any SO or Google results that help. Does anyone see what I'm doing wrong?