Question: MAPLE 2017 fails to draw 3D graph from function generated from Procedures

- I create a function of 2 variables based on Procedures as follows:

Test := proc (a, b)

local z;

if not is({args}, set(numeric)) then return ('procname')('args') end if;

z := a*x^2+b*y^2-3*x+3*y+3;

minimize(z, location)[1]

end proc:

- The function calculates its value if given the specific values of the variables:

Test(2, 2.5) --> 0.975

- But somehow I can't graph this function with the following syntax:

plot3d(Test(a, b), a = 1.0 .. 5.0, b = 2.0 .. 3.0)

- MAPLe 17 issues the following notice: "Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct". And no graphs are drawn

In the same way, MAPLE 15 can plot the graph. Does anyone know what the problem is with MAPLE 2017 and how to fix it?

Sincerely thank