invalid protobuf error running mnist sample

Describe the bug
I built the onnxruntime and the samples according to the instructions. I copied the necessary dlls (libpng16.dll and onnxruntime.dll) into the output folder with mnist.exe. I downloaded the onnx model database here: https://github.com/onnx/models and copied the mnist-8.onnx model as 'model.onnx' into the solution director so I could step through the code with the debugger. When constructing the Ort::Session object, I get the error: "Load model from model.onnx failed:Protobuf parsing failed." and the constructor throws and the program returns 0.

Urgency
Not urgent

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Windows 10
• ONNX Runtime installed from (source or binary): source
• ONNX Runtime version: latest as of today (9 March 2021) I just cloned this repo
• Python version: n/a
• Visual Studio version (if applicable): 2019
• GCC/Compiler version (if compiling from source): MSVC 142, SDK 10.0.183623.0
• CUDA/cuDNN version: n/a
• GPU model and memory: n/a

To Reproduce

• clone this repo
• run build.bat targeting RelWithDebInfo for visual studio 2019:

.\build.bat --config RelWithDebInfo --build_shared_lib --parallel --cmake_generator "Visual Studio 16 2019"

• 0 warnings, 0 errors, 0 of 6 tests failed
• install into default directory (C:\Program Filess (x86)\onnxruntime) by running build on INSTALL in visual studio in admin mode
• cd into samples\c_cxx and run the following commands:

mkdir build
cd build
cmake .. -A x64 -T host=x64 -DLIBPNG_ROOTDIR=<path\to\libpng>

• open samples.sln in visual studio

• build mnist

• manually copy onnxruntime.dll and libpng16.dll into output folder (<path\to\c_cxx\build\MNIST\Debug>)

• clone https://github.com/onnx/models

• copy https://github.com/onnx/models/blob/master/vision/classification/mnist/model/mnist-8.onnx as model.onnx into the mnist sample SolutionDir (<path\to\c_cxx\build\MNIST>)

• set mnist project as startup project in visual studio

• start debugger

• Attach the ONNX model to the issue (where applicable) to expedite investigation.

• Tried to attach! Github wouldn't let me attach the onnx filetype, but the link is above.

Expected behavior
I expected the model to load properly and the program to run.