问python中报错KeyError: -1？

后台python代码为：

print(''.join([id2char[i] for i in pred_ids]))

会报错：

Traceback (most recent call last):

File "D:/迅雷下载/speak/speak/main.py", line 57, in <module>

print(''.join([id2char[i] for i in pred_ids]))

File "D:/迅雷下载/speak/speak/main.py", line 57, in <listcomp>

print(''.join([id2char[i] for i in pred_ids]))

KeyError: -1

请问各位大哥什么原因，怎么解决？