Problem with word completion

1. Let's start nano without a file just for experimentation.
2. Enter 1234  on the first line to have it in the buffer for subsequent
   completing.
3. Enter 1 on the second line, hit {complete}. It completes. {zap}.
4. Enter 12 on the second line, hit {complete}. It completes. {zap}.
5. Enter 123 on the second line, hit {complete}. It writes 'No matches'.

So, when the word is entered without the last character it fails to
complete. I think it's a bug.