Could some help me diagnose what is causing this error? Here is a MVCE:

Code:
. sysuse auto, clear
(1978 Automobile Data)

. crtrees rep78 price mpg, class seed(846)

Classification Trees with learning and test samples (SE rule: 0)
             _yAk6bvkx():  3200  conformability error
             _REAj4BLL():     -  function returned error
                 <istmt>:     -  function returned error
r(3200);
It's happening at

Code:
 - noi mata: _REAj4BLL("`depvar'","`splvar'","`touse'","`learn_smpl'","`test_smpl'", `stop',`rule',"`impurity'","`priors'","`costs'",`vcv', "`jUYYJ5Vnol'","`coeficientes'","`ajustes'","`criterios'","`predicciones'", "`minmax'","`podas
> '","`secuencia'","`clases'")
  = noi mata: _REAj4BLL("rep78","price mpg","__000000","__000004","__000005", 2,0,"gini","__00000X","__00000Y",2147483647, "__000007","__000008","__00000B","__00000D","__00000F", "__00000S","__00000I","__00000T","__00000U")
             _yAk6bvkx():  3200  conformability error
             _REAj4BLL():     -  function returned error
                 <istmt>:     -  function returned error
    }
I am not sure what is going wrong. I get this error whenever the outcome has more than 2 classes. The help file has "The output variable depvar contains J classes or types.", so this should be possible, but there are no classification examples for J>2.