1.4 sigma teleconverter

I've recently bought a sigma 1.4 teleconverter to use with my sigma 120-400 lens (which said it was compatible)only to find there is no 'locking pin' to keep the lens fitted securely to the teleconverter, which given the weight of the lens is a huge problem. has anyone else noticed this problem?

The 120-400mm is HSM so the Sigma teleconverters aren't compatible.

Thanks Mike

I must have read the info on warehouse express incorrectly!