Index: Lib/wave.py
===================================================================
--- Lib/wave.py	(revision 69466)
+++ Lib/wave.py	(working copy)
@@ -462,14 +462,17 @@
         if not self._nframes:
             self._nframes = initlength / (self._nchannels * self._sampwidth)
         self._datalength = self._nframes * self._nchannels * self._sampwidth
-        self._form_length_pos = self._file.tell()
-        self._file.write(struct.pack('<l4s4slhhllhh4s',
+        self._form_length_pos = 4
+        wave_header_format = '<l4s4slhhllhh4s'
+        self._file.write(struct.pack(wave_header_format,
             36 + self._datalength, 'WAVE', 'fmt ', 16,
             WAVE_FORMAT_PCM, self._nchannels, self._framerate,
             self._nchannels * self._framerate * self._sampwidth,
             self._nchannels * self._sampwidth,
             self._sampwidth * 8, 'data'))
-        self._data_length_pos = self._file.tell()
+        self._data_length_pos = (
+                struct.calcsize(wave_header_format) +
+                self._form_length_pos)
         self._file.write(struct.pack('<l', self._datalength))
 
     def _patchheader(self):