Getting BAD Signature when verifying Linux Mint .iso image

I've been attempting to install Linux Mint 18 on a hard drive for a while now, but I continuously run into an error when I try to use gpg --verify. The exact error is as follows:

$ sudo gpg --verify sha256sum.txt.gpg sha256sum.txt
gpg: Signature made Thu 30 Jun 2016 05:13:33 AM MDT using RSA key ID A25BAE09
gpg: BAD signature from "Linux Mint ISO Signing Key <[email protected]>"