ssh-add : Invalid key length

Question

After my mac upgraded automatically, I try ssh-add fail:

>ssh-add
Enter passphrase for /Users/dan/.ssh/id_rsa:
Error loading key "/Users/dan/.ssh/id_rsa": Invalid key length

>ssh -V
OpenSSH_7.6p1, LibreSSL 2.6.2

But how can I fix this issue?

Thanks!