invalid character '\u0081' after top-level value

[GuoxinL]

cocert combine -f cocert0.key -f cocert1.key -o combine.key
Loading PEMs from files...
(Press Enter to continue without decrypt...)
Enter your password for cocert0.key:
Enter your password for cocert1.key:
Decrypting TUF encrypted PEMs...
Enter your master key:
unable to decrypt X509 encoded TUF: invalid character '\u0081' after top-level value

What is the "master key" and what should I fill in, my private key is not password

[GuoxinL]

Step 1:
cocert split --parts 3 --threshold 2 -f admin_sign.key
Splitting private key to Shamir PEMs...
Extracting PEMs to files...
Do you want to encrypt each key using TUF? (y/n) [n]: y
Create new password for cocert0.key key:(123)
Confirm password:(123)
Create new password for cocert1.key key:(456)
Confirm password:(456)
Create new password for cocert2.key key:(789)
Confirm password:(789)
Step 2:
cocert combine -f cocert0.key -f cocert1.key -o combine.key
Loading PEMs from files...
(Press Enter to continue without decrypt...)
Enter your password for cocert0.key:(123)
Enter your password for cocert1.key:(456)
Decrypting TUF encrypted PEMs...
Enter your master key: (???)
unable to decrypt X509 encoded TUF: invalid character '\u0081' after top-level value