For tag fails in template

What is wrong?

My view function:
def project(request, project_id):

try:

project_id = int(project_id)

except ValueError:

raise Http404

myProject = get_object_or_404(Project, id = project_id)
events = Event.objects.filter(project = myProject)

return render_to_response('project.html',

context_instance=RequestContext(request, {'project':
myProject, 'events': events}))

And in the template I do:
<div id="sidebar">
<ul>
{% for event in events %}
<li><a href="{% url event event.pk %}">{{ event.title }}</a></li>
{% empty %}
<p>{% trans "There are no events yet." %}</p>
{% endfor %}
</ul>
</div>

If there are no events, everything is fine. If some event exists, it
shows an error:

Caught NoReverseMatch while rendering: Reverse for 'event' with
arguments '(1,)' and keyword arguments '{}' not found.

and points to the for tag.

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