diff --git a/client/handlers.go b/client/handlers.go
index 089c103..145742f 100644
--- a/client/handlers.go
+++ b/client/handlers.go
@@ -125,7 +125,12 @@ func (conn *Conn) h_PART(line *Line) {
 	ch := conn.GetChannel(line.Args[0])
 	n := conn.GetNick(line.Nick)
 	if ch != nil && n != nil {
-		ch.DelNick(n)
+		if _, ok := ch.Nicks[n]; ok {
+			ch.DelNick(n)
+		} else {
+			conn.error("irc.PART(): nick %s is not on channel %s",
+				line.Nick, line.Args[0])
+		}
 	} else {
 		conn.error("irc.PART(): buh? PART of channel %s by nick %s",
 			line.Args[0], line.Nick)
diff --git a/client/handlers_test.go b/client/handlers_test.go
index 775f55e..9a3a8fa 100644
--- a/client/handlers_test.go
+++ b/client/handlers_test.go
@@ -227,14 +227,9 @@ func TestPART(t *testing.T) {
 	}
 
 	// Test error states.
-	// TODO(fluffle): BUG?
-	// user1 is still a known Nick (on #test2), but receiving another PART
-	// for user1 from #test1 (or, in fact, a PART on any known Channel, whether
-	// they are on it or not) won't cause an error because h_PART doesn't
-	// check whether the Nick is on the Channel before calling DelNick().
-	// The Nick and Channel structs should expose more of the state info.
+	// Part a known user from a known channel they are not on.
 	c.h_PART(parseLine(":user1!ident1@host1.com PART #test1 :Bye!"))
-	c.ExpectNoErrors()
+	c.ExpectError()
 
 	// Part an unknown user from a known channel.
 	c.h_PART(parseLine(":user2!ident2@host2.com PART #test1 :Bye!"))