diff --git a/io.c b/io.c
index eebaa09825..aa2d49a6b2 100644
--- a/io.c
+++ b/io.c
@@ -1294,11 +1294,13 @@ rb_io_set_sync(VALUE io, VALUE sync)
  *     ios.fsync   => 0 or nil
  *
  *  Immediately writes all buffered data in <em>ios</em> to disk.
- *  Returns <code>nil</code> if the underlying operating system does not
- *  support <em>fsync(2)</em>. Note that <code>fsync</code> differs from
+ *  Note that <code>fsync</code> differs from
  *  using <code>IO#sync=</code>. The latter ensures that data is flushed
  *  from Ruby's buffers, but doesn't not guarantee that the underlying
  *  operating system actually writes it to disk.
+ *
+ *  <code>NotImplementedError</code> is raised
+ *  if the underlying operating system does not support <em>fsync(2)</em>. 
  */
 
 static VALUE
@@ -1325,8 +1327,9 @@ rb_io_fsync(VALUE io)
  *     ios.fdatasync   => 0 or nil
  *
  *  Immediately writes all buffered data in <em>ios</em> to disk.
- *  Returns <code>nil</code> if the underlying operating system does not
- *  support <em>fdatasync(2)</em>.
+ *
+ *  <code>NotImplementedError</code> is raised
+ *  if the underlying operating system does not support <em>fdatasync(2)</em>. 
  */
 
 static VALUE