This is an often heard problem with Prime - Labels.
The cause for the error (the error message is misleading, to say the least) is that your variable "v" at the abscissa and the parameter "v" in your function P.aero at the ordinate have different Labels.
Solution is to either change the label of "v" at the abscissa from "none" to "Variable" (as in the attched) or change the label of the parameter "v" at the ordinate from "Variable" to "none".