diff --git a/proxy/lookup.go b/proxy/lookup.go
index c6246c2a4..8fe385eff 100644
--- a/proxy/lookup.go
+++ b/proxy/lookup.go
@@ -32,6 +32,8 @@ func (p *Proxy) lookupIPAddr(host string, qtype uint16, ch chan *lookupResult) {
 	ch <- &lookupResult{d.Res, err}
 }
 
+// ErrEmptyHost is returned by LookupIPAddr when the host is empty and can't be
+// resolved.
 const ErrEmptyHost = errors.Error("host is empty")
 
 // LookupIPAddr resolves the specified host IP addresses