diff --git a/libguile/goops.c b/libguile/goops.c
index 61f81e6c5..1c61d10ae 100644
--- a/libguile/goops.c
+++ b/libguile/goops.c
@@ -2741,9 +2741,10 @@ create_port_classes (void)
 {
   long i;
 
-  scm_port_class = (SCM *) scm_malloc (3 * 256 * sizeof (SCM));
-  for (i = 0; i < 3 * 256; ++i)
-    scm_port_class[i] = 0;
+  /* Allocate 3 times the maximum number of port types so that input ports,
+     output ports, and in/out ports can be stored at different offsets.  See
+     `SCM_IN_PCLASS_INDEX' et al.  */
+  scm_port_class = scm_calloc (3 * SCM_I_MAX_PORT_TYPE_COUNT * sizeof (SCM));
 
   for (i = 0; i < scm_numptob; ++i)
     scm_make_port_classes (i, SCM_PTOBNAME (i));
diff --git a/libguile/objects.h b/libguile/objects.h
index 68996d2a0..9b2a0ed5a 100644
--- a/libguile/objects.h
+++ b/libguile/objects.h
@@ -3,7 +3,7 @@
 #ifndef SCM_OBJECTS_H
 #define SCM_OBJECTS_H
 
-/* Copyright (C) 1996,1999,2000,2001, 2003, 2006, 2008 Free Software Foundation, Inc.
+/* Copyright (C) 1996,1999,2000,2001, 2003, 2006, 2008, 2009 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -171,9 +171,9 @@ typedef struct scm_effective_slot_definition {
 #define SCM_CMETHOD_ENV(cmethod)  SCM_CAR (cmethod)
 
 /* Port classes */
-#define SCM_IN_PCLASS_INDEX    0x000
-#define SCM_OUT_PCLASS_INDEX   0x100
-#define SCM_INOUT_PCLASS_INDEX 0x200
+#define SCM_IN_PCLASS_INDEX       0
+#define SCM_OUT_PCLASS_INDEX      SCM_I_MAX_PORT_TYPE_COUNT
+#define SCM_INOUT_PCLASS_INDEX    (2 * SCM_I_MAX_PORT_TYPE_COUNT)
 
 /* Plugin proxy classes for basic types. */
 SCM_API SCM scm_metaclass_standard;
diff --git a/libguile/ports.c b/libguile/ports.c
index 964d4d113..c961ce3ae 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2003, 2004, 2006, 2007, 2008 Free Software Foundation, Inc.
+/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2003, 2004, 2006, 2007, 2008, 2009 Free Software Foundation, Inc.
  * 
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -134,7 +134,7 @@ scm_make_port_type (char *name,
 		    void (*write) (SCM port, const void *data, size_t size))
 {
   char *tmp;
-  if (255 <= scm_numptob)
+  if (SCM_I_MAX_PORT_TYPE_COUNT - 1 <= scm_numptob)
     goto ptoberr;
   SCM_CRITICAL_SECTION_START;
   tmp = (char *) scm_gc_realloc ((char *) scm_ptobs,
diff --git a/libguile/ports.h b/libguile/ports.h
index 48bc643db..5e42a3468 100644
--- a/libguile/ports.h
+++ b/libguile/ports.h
@@ -3,7 +3,7 @@
 #ifndef SCM_PORTS_H
 #define SCM_PORTS_H
 
-/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2003, 2004, 2006, 2008 Free Software Foundation, Inc.
+/* Copyright (C) 1995,1996,1997,1998,1999,2000,2001, 2003, 2004, 2006, 2008, 2009 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -162,6 +162,9 @@ SCM_INTERNAL SCM scm_i_port_weak_hash;
 #define SCM_DECCOL(port)  	{if (SCM_COL (port) > 0) SCM_COL (port) -= 1;}
 #define SCM_TABCOL(port)  	{SCM_COL (port) += 8 - SCM_COL (port) % 8;}
 
+/* Maximum number of port types.  */
+#define SCM_I_MAX_PORT_TYPE_COUNT  256
+
 
 
 /* port-type description.  */