Why does this arithmetic return as an error?

local hp = tonumber(plr.Stats.Health.Value) * (tonumber(plr.Stats.Hpt.Value) * 0.07 + 1)

Thats the line that returns as an error, and I get "attempt to perform arithmetic (add) on number and nil" as the error. I'm not sure what nil means, because the value does exist. Can anyone help?