opus - Oct 2013 - Unexpected opus_encode results when doubelling frame size

Hello!

I have been using

int iByteSizeEncoded = opus_encode(m_enc, 
m_ShortRawInput,960,m_EncodedBytes, 4000);

iByteSizeEncoded was 120 as expected.

Now when I double the frame size...

int iByteSizeEncoded = opus_encode(m_enc, 
m_ShortRawInput,960*2,m_EncodedBytes, 4000);

... iByteSizeEncoded is not 240 as expected, but 239.

This seems really odd to me since 239 bytes could not even be converted 
to shorts.

Can somebody shed light on what is going on here?
Apart from the frame size I did not change anything.

Thank you.