Confusing error for uninferred type argument

I am running the following in the playground REPL:

```
panic("hi")
```

The error I get is very unclear:

```
[error] interactive.effekt:7:14: Cannot infer type argument R, maybe consider annotating it?
[error] module1.effekt:7:14: Cannot infer type argument R, maybe consider annotating it?
```

I don't know where `R` comes from. I thought maybe `panic` could be polymorphic, but explicit type arguments don't work:

```
panic[String]("hi")
```

and annotating in the obvious way doesn't parse

```
panic("hi"): String
```