r - Random forest k fold cross validation error for list class -

i'm trying k-fold cross validation rf:

fit3 <- rfcv(x, y, data=trainset1, controls=cforest_unbiased, cv.fold=10)   testset1 <- as.data.frame(testset1) pred1    <- predict(fit3, testset1, oob=true) cor.test(pred1, testset$swl) 

i encounter error:

error in usemethod("predict") :  no applicable method 'predict' applied object of class "list" 

i'm sure testset1 not list.