Sitecore Experience Commerce - Engine could not load type

During an upgrade of Sitecore Experience Commerce (from update 1 to update 2), I was getting the following error message on a failed bootstrap of the commerce engine:

Could not load type 'Sitecore.Commerce.Plugin.Workflow.Workflow' from assembly 'Sitecore.Commerce.ServiceProxy, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null'.

As I had customized the commerce engine, I had to upgrade the project from the latest SDK manually (as opposed to simply deploying the pre-compiled version). This involved update plugin references, merging environment JSON configuration files and adding new DLL references. In this case, I had missed adding the new reference to Sitecore.Commerce.Plugin.Workflow and it caused the bootstrap to fail.

Adding a reference to the Sitecore.Commerce.Plugin.Workflow inside the Sitecore.Commerce.Engine project, re-deploying then allowed the bootstrap to complete.