Error when calling terminateEngineClient

I am running a MATLAB engine from C++. Everything works until I want to finish my simulation and call matlab::engine::terminateEngineClient() to close the instance of MATLAB running in the background. I then get this error in the terminal:

terminate called after throwing an instance of 'foundation::core::except::Exception<MATLAB::engineAPI::MatlabProcessNotTerminated, engine_api::EngineException, void>'
  what():  MATLAB process cannot be terminated.

I am running this in CentOS 7 using MATLAB 2019a. Google does not return any results regarding this error.

I know there are a lot more factors that influence this (compiler version, etc.) but I don't know what to look for. Please let me know what information to include.