diff --git a/libguile/ports-internal.h b/libguile/ports-internal.h
index 2b2ccc4fb..7b2bd68b2 100644
--- a/libguile/ports-internal.h
+++ b/libguile/ports-internal.h
@@ -42,6 +42,7 @@ typedef enum scm_t_port_type_flags {
 /* port-type description.  */
 struct scm_t_port_type
 {
+  scm_t_bits tag;
   char *name;
   int (*print) (SCM exp, SCM port, scm_print_state *pstate);
 
diff --git a/libguile/ports.c b/libguile/ports.c
index c2b79ab0e..d21786dc3 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -250,9 +250,10 @@ scm_make_port_type (char *name,
 {
   scm_t_port_type *desc;
 
-  desc = scm_allocate_pointerless (SCM_I_CURRENT_THREAD, sizeof (*desc));
+  desc = scm_allocate_tagged (SCM_I_CURRENT_THREAD, sizeof (*desc));
   memset (desc, 0, sizeof (*desc));
 
+  desc->tag = scm_tc7_port_type;
   desc->name = name;
   desc->print = scm_port_print;
   desc->c_read = read;
diff --git a/libguile/scm.h b/libguile/scm.h
index 5509d239e..c887424ea 100644
--- a/libguile/scm.h
+++ b/libguile/scm.h
@@ -487,7 +487,7 @@ typedef uintptr_t scm_t_bits;
 #define scm_tc7_vm_cont		0x47
 #define scm_tc7_bytevector	0x4d
 #define scm_tc7_thread		0x4f
-#define scm_tc7_unused_55	0x55
+#define scm_tc7_port_type	0x55
 #define scm_tc7_unused_57	0x57
 #define scm_tc7_array		0x5d
 #define scm_tc7_bitvector	0x5f