Testfor Player Health 0 [closed]

I am trying to testfor @a health at 0 (when the player dies) using the /scoreboard command. The commands I was using do not seem to be working; the commandblock output is "cannot be found." I added an objective:

/scoreboard objectives add health health

And I added the testfor:

/testfor @p[score_health_min=0,score_health=0]

Yet this is brings up the error:

Entity '@p[score_health_min=0,score_health=0]' cannot be found.

How can this be done correctly?

While the above answer is valid, I believe this would be faster and more efficient.

scoreboard objectives add dieTime stat.timeSinceDeath
execute @a[score_dieTime=0] ~ ~ ~ say I am dead!