[R] Partial dependence plot in randomForest package (all flat responses)

[Oritteropus]

Hi,
I'm trying to make a partial plot with package randomForest in R. After I
perform my random forest object I type

partialPlot(data.rforest, pred.data=act2, x.var=centroid, "C") 

where data.rforest is my randomforest object, act2 is the original dataset,
centroid is one of the predictor and C is one of the classes in my response
variable. 
Whatever predictor or response class I try I always get a plot with a
straight line (a completely flat response). Similarly, If I set a
categorical variable as predictor, I get a barplot with all the bar with the
same height. I suppose I'm doing something wrong here because all other
analysis on the same rforest object seem correct (e.g. varImp or MDSplot).
Is it possible it is related to some option set in random forest object? Can
somebody see the problem here?
Thanks for your time



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