Given a list of letters, finding all possible words in a ternary search tree

Question

I wrote a program to find all possible words with a list of given letters in a ternary search tree. The output is a sorted set of all words. Following is the python code for finding the words:

def _find_words(self, node: Node, letters: str, buffer=''):
    if node is None:
        return
    for c in letters:
        if node.key < c:
            self._find_words(node.high, letters, '')
        elif node.key > c:
            self._find_words(node.low, letters, '')
        else:
            buffer += node.key
            if node.isEnd:
                self.words.add(buffer)
                self._find_words(node.equal, 
                                letters.replace(node.key, '', 1), buffer)

The tree contains the following words: cat, acts, act, actor, at, bug, cats, up, actors, upper

I get the following output for the letters 'abugpscort':

['acts', 'cats', 'cat', 'act', 'bug', 'ors', 'up', 'or', 't']

whereas the output should be:

['actors', 'actor', 'acts', 'cats', 'cat', 'act', 'bug', 'up', 'at']

How can I fix this?