Can't use the AREF pin

Hey, I would like to use an external reference on the AREF pin but I can't set the analog reference to EXTERNAL (I saw somewhere that the Due is always EXTERNAL but it's not said anywhere on the documentation). Here's the error message:

Arduino : 1.6.9 (Windows 10), Carte : "Arduino Due (Programming Port)"

C:\Users\chaisePliante\Desktop\voltm_tre\voltm_tre.ino: In function 'void setup()':

voltm_tre:5: error: invalid conversion from 'int' to 'eAnalogReference {aka _eAnalogReference}' [-fpermissive]



In file included from C:\Users\chaisePliante\AppData\Local\Arduino15\packages\arduino\hardware\sam\1.6.8\cores\arduino/Arduino.h:205:0,

                 from sketch\voltm_tre.ino.cpp:1:

C:\Users\chaisePliante\AppData\Local\Arduino15\packages\arduino\hardware\sam\1.6.8\cores\arduino/wiring_analog.h:40:13: error:   initializing argument 1 of 'void analogReference(eAnalogReference)' [-fpermissive]

 extern void analogReference( eAnalogReference ulMode ) ;


exit status 1
invalid conversion from 'int' to 'eAnalogReference {aka _eAnalogReference}' [-fpermissive]

Do you see what went wrong? Thanks

Looks like it is expecting an enum or something like that.