Bug: Poor error message for invalid use of `then` keyword

### Input Code
```coffee
then
```

### Expected Behavior
```
[stdin]:1:1: error: unexpected then
then
^^^^
```

### Current Behavior
```
TypeError: Cannot read properties of undefined (reading '2')
```

### Context

This is very minor but thought I'd point it out.

https://coffeescript.org/#try:then

* CoffeeScript version: 2.7.1
