--- a/libmu_scm/mu_body.c	
+++ a/libmu_scm/mu_body.c	
@@ -40,7 +40,7 @@ mu_scm_body_mark (SCM body_smob)
   return mbp->msg;
 }
 
-static scm_sizet
+static size_t
 mu_scm_body_free (SCM body_smob)
 {
   struct mu_body *mbp = (struct mu_body *) SCM_CDR (body_smob);
--- a/libmu_scm/mu_dbgport.c	
+++ a/libmu_scm/mu_dbgport.c	
@@ -79,7 +79,7 @@ _mu_debug_port_close (SCM port)
   return 0;
 }
 
-static scm_sizet
+static size_t
 _mu_debug_port_free (SCM port)
 {
   _mu_debug_port_close (port);
--- a/libmu_scm/mu_mailbox.c	
+++ a/libmu_scm/mu_mailbox.c	
@@ -37,7 +37,7 @@ mu_scm_mailbox_mark (SCM mailbox_smob)
   return SCM_BOOL_F;
 }
 
-static scm_sizet
+static size_t
 mu_scm_mailbox_free (SCM mailbox_smob)
 {
   struct mu_mailbox *mum = (struct mu_mailbox *) SCM_CDR (mailbox_smob);
--- a/libmu_scm/mu_message.c	
+++ a/libmu_scm/mu_message.c	
@@ -39,7 +39,7 @@ mu_scm_message_mark (SCM message_smob)
   return mum->mbox;
 }
 
-static scm_sizet
+static size_t
 mu_scm_message_free (SCM message_smob)
 {
   struct mu_message *mum = (struct mu_message *) SCM_CDR (message_smob);
--- a/libmu_scm/mu_mime.c	
+++ a/libmu_scm/mu_mime.c	
@@ -35,7 +35,7 @@ mu_scm_mime_mark (SCM mime_smob)
   return mum->owner;
 }
 
-static scm_sizet
+static size_t
 mu_scm_mime_free (SCM mime_smob)
 {
   struct mu_mime *mum = (struct mu_mime *) SCM_CDR (mime_smob);
--- a/libmu_scm/mu_port.c	
+++ a/libmu_scm/mu_port.c	
@@ -147,7 +147,7 @@ mu_port_close (SCM port)
   return 0;
 }
 
-static scm_sizet
+static size_t
 mu_port_free (SCM port)
 {
   struct mu_port *mp = MU_PORT (port);