Frank Bryce's Death - query

[mooseming]

I've been reviewing DD's HRX identification and have come across an 
*error* which is bugging me and for which I can see no *obvious* 
implications.

DD says Voldy got the idea of using Nagini as a HRX after Voldy used 
her to "kill an old muggle man" for which I take it he meant Frank 
Bryce in the opening of GoF.  Yet DD *knows* Voldy used his wand for 
that murder as Harry reports his appearance during the Priori 
Incantatem incident.

Is there another death to which DD is refering that I've forgotten 
about? If not why would DD make this error?

Regards
Jo