Undefined Columns Selected for R Classification Tree

Question

I am working with the popular tumor dataset in R, trying to create a classification tree that predicts the classification of the Diagnosis variable. However, when I run my tree() statement on my training dataset (which contains only numeric variables), I get the following error:

tumor.tree <- tree(Diagnosis ~ ., data = tumor.train)

Error in `[.data.frame`(frame, predictors) : undefined columns selected

I have seen that this sometimes happens when column names are numbers, but none of my column names are numbers in this example. I am not sure what could be the issue in my call to tree(), and would appreciate any advice. Thank you!