Problem with integer variable and vector range

Hello,

"OutputNumber" is a Std_logic_Vector(5 downto 0). It is a vector that store the number of output (eg.: "000000" means no outputs; "01010" means 10 outputs and so on....)

The integer variable "Int" store the equivalent integer value: No outputs: Int:=0; 10 outputs: Int:=10 and so on.....

"Temp" is a 38bit vector, but i need to copy only a range to "MyVector"

Int <= conv_integer(OutputNumber);

MyVector(37 downto (38-Int)) <= Temp(37 downto (38-Int));

Quartus Compiler Error:

Error (10454): VHDL syntax error: right bound of range must be a constant