Incorrect error?

["ldries46" <bertus.dries@planet.nl>]

I'm using Ada.Numerics.Generic_Real_Arrays. While Compiling I got the 
following error message:

65:21     expected type "Ada.Numerics.Generic_Real_Arrays.Real_Vector" from 
instance at line 6
65:21     found type "Ada.Numerics.Generic_Real_Arrays.Real_Matrix" from 
instance at line 6

As can be seen Cross, a and b declared as Real_Vector .
What do I do wrong?

L. Dries

Code:

package Float_Arrays is new Ada.Numerics.Generic_Real_Arrays (float);
use Float_Arrays;

function Volume (ObjID : integer; Obj : ptrS_Object) return float is
   ok     : boolean;
   n      : integer := 1;
   Volume : float;
   a      : Real_Vector(0 .. 2);
   b      : Real_Vector(0 .. 2);
   X0     : Real_Vector(0 .. 2);
   X1     : Real_Vector(0 .. 2);
   X2     : Real_Vector(0 .. 2);
   cross  : Real_Vector(0 .. 2);
   nA     : Real_Vector(0 .. 2);
   p      : ptrNode_Object;
   TempS  : ptrS_Object;
   TempT  : ptrTriangle_Object;
begin
   TempS := Obj;
   while n /= ObjID loop
      n := n + 1;
      TempS := TempS.next;
      if TempS = null then
         declare
            NO_NODE_OBJECT : exception;
         begin
            raise NO_NODE_OBJECT;
         end;
      end if;
   end loop;
   n := TempS.PosFaces(0);
   TempT := stT_Object;
   ok := TempT = null;
   while not ok loop
      if TempT.nr = n then
         ok := true;
      else
         TempT := TempT.next;
         ok := TempT = null;
      end if;
   end loop;
   while n < TempS.PosFaces(1) loop
      p := GetPoint(TempT.P1);
      X0(0) := p.pRectangular(x);
      X0(1) := p.pRectangular(y);
      X0(2) := p.pRectangular(z);
      p := GetPoint(TempT.P2);
      X1(0) := p.pRectangular(x);
      X1(1) := p.pRectangular(y);
      X1(2) := p.pRectangular(z);
      p := GetPoint(TempT.P3);
      X2(0) := p.pRectangular(x);
      X2(1) := p.pRectangular(y);
      X2(2) := p.pRectangular(z);
      a := X1 - X0; -- VecSub( a, X1, X0 );
      b := X2 - X0; -- VecSub( b, X2, X0 );
     Cross := a * b; -- VecCross( a, b, Cross ); 
<---------------- line 65 the cursor is standing just before *
      nA := Cross * 0.5; -- VecMultScalar( nA, Cross, 0.5 );
      -- Volume = Volume + fabs(VecDot( X0, nA ));
      Volume := Volume + X0(0)*nA(0) + X0(1)*nA(1) + X0(2)*nA(2);
      n := n + 1;
   end loop;
   Volume := Volume/3.0;
   return Volume;
end Volume;