If you are using a test runner that runs code in a browser-based environment, you may be using SystemJS in order to bundle your React code.
Prior to enzyme 3.0 there were some issues with conditional requires that were used to maintain backwards compatibility with React versions. With enzyme 3.0+, this should no longer be an issue. If it is, please file a GitHub issue or make a PR to this documentation with instructions on how to set it up.