Invariant Violation

Not sure what causes this error, I'm not able to build using this plugin:

```
Invariant Violation
at invariant (/my_project/node_modules/prepack/src/invariant.js:15:15)
at new Serializer (/my_project/node_modules/prepack/src/serializer/serializer.js:64:5)
at prepack (/my_project/node_modules/prepack/src/prepack-standalone.js:36:20)
at Compilation.compilation.plugin (/my_project/node_modules/prepack-webpack-plugin/src/PrepackPlugin.js:58:35)
at Compilation.applyPluginsAsyncSeries (/my_project/node_modules/tapable/lib/Tapable.js:142:13)
at self.applyPluginsAsync.err (/my_project/node_modules/webpack/lib/Compilation.js:631:10)
at Compilation.applyPluginsAsyncSeries (/my_project/node_modules/tapable/lib/Tapable.js:131:46)
at sealPart2 (/my_project/node_modules/webpack/lib/Compilation.js:627:9)
at Compilation.applyPluginsAsyncSeries (/my_project/node_modules/tapable/lib/Tapable.js:131:46)
at Compilation.seal (/my_project/node_modules/webpack/lib/Compilation.js:575:8)
```

How can I provide more info?

Start by reproducing the build failure with the reported prepack-webpack-plugin stack trace. Read prepack-webpack-plugin/src/PrepackPlugin.js and the referenced Prepack serializer entry points to determine what input triggers the invariant. Done means identifying the cause and documenting or fixing the failure with a reproducible test case.

Written by the indexing model from the issue text.