Silent failure when staticPaths points to a file instead of a directory

[viking]

Given this app:

app <- list(
  call = function(req) {
    list(status = 404L,
         headers = list('Content-Type' = 'text/plain'),
         body = "Not found")
  },
  staticPaths = list(
    "/foo" = "foo.html"
  ),
  staticPathOptions = staticPathOptions(fallthrough = TRUE)
)
s <- startServer('127.0.0.1', 31337, app)

Any attempts to fetch a subpath under /foo will fail silently:

$ curl -v http://localhost:31337/foo/bar
*   Trying 127.0.0.1:31337...
* TCP_NODELAY set
* Connected to localhost (127.0.0.1) port 31337 (#0)
> GET /foo/bar HTTP/1.1
> Host: localhost:31337
> User-Agent: curl/7.66.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 500 Internal Server Error
< Date: Fri, 08 May 2020 02:20:02 GMT
< Content-Length: 26
< 
500 Internal Server Error
* Connection #0 to host localhost left intact

There is no error in the R console when this request is made.

This may not be a bug per se, but if specifying a file instead of a directory in staticPaths is not allowed, I would have expected some sort of error telling me so. Let me know if I can provide any further information.