diff --git a/libguile/ports.c b/libguile/ports.c
index a3d882730..49224beb4 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -418,7 +418,8 @@ scm_close_port (port)
 
   port = SCM_COERCE_OUTPORT (port);
 
-  SCM_ASSERT (SCM_NIMP (port) && SCM_PORTP (port), port, SCM_ARG1, s_close_port);
+  SCM_ASSERT (SCM_NIMP (port) && SCM_OPPORTP (port), port, SCM_ARG1,
+	      s_close_port);
   if (SCM_CLOSEDP (port))
     return SCM_BOOL_F;
   i = SCM_PTOBNUM (port);