Cannot set onerror handler

[halloleo]

When I try to set an onerror handler on an httpRequest I get a type mismatch.

Here the details. The Nim code is:

import dom
import ajax

proc makeRequest(url:cstring) =
  var httpRequest = newXMLHttpRequest()

  proc alertContents(e:Event) =
    if httpRequest.readyState == rsDONE:
      if httpRequest.status == 200:
        window.alert(httpRequest.responseText)
      else:
        window.alert("There was a problem with the request.")
  proc alertError(e:Event) =
        window.alert("There was a FUNDAMENTAL problem with the request.")

  httpRequest.onreadystatechange = alertContents
  httpRequest.onerror = alertError
  httpRequest.open("GET", url);
  httpRequest.send();

document.getElementById("ajaxButton").onclick = proc(e:Event) =
  makeRequest("test.html")

The full error message of the compiler is:

Error: type mismatch: got <XMLHttpRequest, proc (e: Event){.noSideEffect, gcsafe, locks: 0.}>
but expected one of: 
macro `.=`(obj: JsObject; field, value: untyped): untyped
  first type mismatch at position: 1
  required type for obj: JsObject
  but expression 'httpRequest' is of type: XMLHttpRequest
macro `.=`[K: cstring; V](obj: JsAssoc[K, V]; field: untyped; value: V): untyped
  first type mismatch at position: 1
  required type for obj: JsAssoc[.=.K, .=.V]
  but expression 'httpRequest' is of type: XMLHttpRequest