Question: Optimization of external one

Have a piece of code:with(Optimization):

with(ExternalCalling):
MyTargetMultArgsFunctionExt := define_external('MyTargetMultArgsFunctionExt', 'MAPLE', x::(float[8]), y::(float[8]), 'LIB' = "maple_library.dll");
G := proc (x, y) options operator, arrow; MyTargetMultArgsFunctionExt(x, y) end proc;
dMyTargetMultArgsFunctionExtr0 := proc (x, y) options operator, arrow; x^2 end proc;
Minimize(G)

After which i obtain warning:

Attention: in the procedure are external procedures
{MyTargetMultArgsFunctionExt}
of which the gradients have been computed and
are now available under the names:
{MyTargetMultArgsFunctionExt .. dMyTargetMultArgsFunctionExtr1}

How to overcome it? AS you can see i firstly calculate derivative "straightforward" but it doesn't help.