--- src/sequence.c.orig	2012-12-19 02:08:31.000000000 -0500
+++ src/sequence.c	2012-12-19 02:16:05.000000000 -0500
@@ -7607,6 +7607,18 @@
 
   CHECK_KEY_ARGUMENT (key);
 
+  if (EQ (caller, Qsubsetp))
+    {
+      if (NILP (liszt1))
+	{
+	  return Qt;
+	}
+      if (NILP (liszt2))
+	{
+	  return Qnil;
+	}
+    }
+
   if (NILP (liszt1) && intersectionp)
     {
       return Qnil;