As long as he doesn't lead with the crown of his helmet then its not. Runner is not defenseless. So you could only call
Targeting and Initiating Contact With the Crown of the Helmet
9-1-3
2014 PenaltyA.R.
No player shall target and initiate contact against an opponent with the crown (top) of his helmet. When in question, it is a foul. (Rule 9-6) (A.R. 9-1-3-I)
Otherwise I got nothing!