Error in babel when using "let"

We are getting an error with our pr danger file after we added in one new line.
```
let labels = let labels = danger.github.issue.labels.map(l => l.name);
```

But if I use "var" everything works fine

This is the error that occurs when using "let"

```
/app/danger-0.ffgmk6xn2dt.js: Cannot read property 'bindings' of null
TypeError: /app/danger-0.ffgmk6xn2dt.js: Cannot read property 'bindings' of null
at Scope.moveBindingTo (/app/node_modules/babel-core/node_modules/babel-traverse/lib/scope/index.js:960:13)
at BlockScoping.updateScopeInfo (/app/node_modules/babel-plugin-transform-es2015-block-scoping/lib/index.js:364:17)
at BlockScoping.run (/app/node_modules/babel-plugin-transform-es2015-block-scoping/lib/index.js:330:12)
at PluginPass.BlockStatementSwitchStatementProgram (/app/node_modules/babel-plugin-transform-es2015-block-scoping/lib/index.js:70:24)
at newFn (/app/node_modules/babel-core/node_modules/babel-traverse/lib/visitors.js:234:21)
at NodePath._call (/app/node_modules/babel-core/node_modules/babel-traverse/lib/path/context.js:67:18)
at NodePath.call (/app/node_modules/babel-core/node_modules/babel-traverse/lib/path/context.js:42:17)
at NodePath.visit (/app/node_modules/babel-core/node_modules/babel-traverse/lib/path/context.js:98:12)
at TraversalContext.visitQueue (/app/node_modules/babel-core/node_modules/babel-traverse/lib/context.js:136:16)
at TraversalContext.visitSingle (/app/node_modules/babel-core/node_modules/babel-traverse/lib/context.js:96:19)
```
I found a reference the exact error and backtrace here, which seems to imply a mismatch of babel core versions and plugins versions.

Reference: https://github.com/babel/babel/issues/6186