diff --git a/libguile/ChangeLog b/libguile/ChangeLog
index 2d0316f03..6afd8924e 100644
--- a/libguile/ChangeLog
+++ b/libguile/ChangeLog
@@ -1,3 +1,7 @@
+1999-10-26  Gary Houston  <ghouston@freewire.co.uk>
+
+	* strports.c (st_end_input): avoid dubious pointer arithmetic.
+
 1999-10-24  Gary Houston  <ghouston@freewire.co.uk>
 
 	* Move the responsibility for resetting port buffers from the
diff --git a/libguile/strports.c b/libguile/strports.c
index d76a6009c..19e2ba1b2 100644
--- a/libguile/strports.c
+++ b/libguile/strports.c
@@ -149,12 +149,11 @@ static void
 st_end_input (SCM port, int offset)
 {
   scm_port *pt = SCM_PTAB_ENTRY (port);
-  const unsigned char *pos = pt->read_pos - offset;
-
-  if (pos < pt->read_buf)
-    scm_misc_error ("st_end_input", "negative position", SCM_EOL);
   
-  pt->write_pos = (unsigned char *) pt->read_pos = pos;
+  if (pt->read_pos - pt->read_buf < offset)
+    scm_misc_error ("st_end_input", "negative position", SCM_EOL);
+
+  pt->write_pos = (unsigned char *) pt->read_pos = pt->read_pos - offset;
   pt->rw_active = SCM_PORT_NEITHER;
 }