diff --git a/libguile/ports.c b/libguile/ports.c
index 144daef3f..8fe8dbe0d 100644
--- a/libguile/ports.c
+++ b/libguile/ports.c
@@ -2614,15 +2614,16 @@ scm_puts (const char *s, SCM port)
 static void
 scm_i_write_bytes_unlocked (SCM port, SCM src, size_t start, size_t count)
 {
-  size_t written;
+  size_t written = 0;
   scm_t_ptob_descriptor *ptob = SCM_PORT_DESCRIPTOR (port);
 
   assert (count <= SCM_BYTEVECTOR_LENGTH (src));
   assert (start + count <= SCM_BYTEVECTOR_LENGTH (src));
 
-  written = ptob->write (port, src, start, count);
+  do
+    written += ptob->write (port, src, start + written, count - written);
+  while (written < count);
 
-  /* FIXME: Allow short writes?  */
   assert (written == count);
 }
 
@@ -2697,13 +2698,10 @@ scm_c_write_bytes_unlocked (SCM port, SCM src, size_t start, size_t count)
     {
       /* Our write would overflow the buffer.  Flush buffered bytes (if
          needed), then write our bytes with just one syscall.  */
-      size_t written;
-
       if (write_buf->cur < write_buf->end)
         scm_i_write_unlocked (port, write_buf);
 
-      written = SCM_PORT_DESCRIPTOR (port)->write (port, src, start, count);
-      assert (written == count);
+      scm_i_write_bytes_unlocked (port, src, start, count);
     }
 
   return count;
diff --git a/libguile/r6rs-ports.c b/libguile/r6rs-ports.c
index fb821bb18..4b2df9229 100644
--- a/libguile/r6rs-ports.c
+++ b/libguile/r6rs-ports.c
@@ -887,29 +887,18 @@ static size_t
 custom_binary_output_port_write (SCM port, SCM src, size_t start, size_t count)
 #define FUNC_NAME "custom_binary_output_port_write"
 {
-  size_t written;
   struct custom_binary_port *stream = (void *) SCM_STREAM (port);
+  size_t written;
+  SCM result;
 
-  /* Since the `write' procedure of Guile's ports has type `void', it must
-     try hard to write exactly SIZE bytes, regardless of how many bytes the
-     sink can handle.  */
-  written = 0;
-  while (written < count)
-    {
-      long int c_result;
-      SCM result;
+  result = scm_call_3 (stream->write, src, scm_from_size_t (start),
+                       scm_from_size_t (count));
 
-      result = scm_call_3 (stream->write, src,
-			   scm_from_size_t (start + written),
-			   scm_from_size_t (count - written));
-
-      c_result = scm_to_long (result);
-      if (c_result < 0 || (size_t) c_result > (count - written))
-	scm_wrong_type_arg_msg (FUNC_NAME, 0, result,
-				"R6RS custom binary output port `write!' "
-				"returned a incorrect integer");
-      written += c_result;
-    }
+  written = scm_to_size_t (result);
+  if (written > count)
+    scm_wrong_type_arg_msg (FUNC_NAME, 0, result,
+                            "R6RS custom binary output port `write!' "
+                            "returned a incorrect integer");
 
   return written;
 }