R Reference Class issue

I am trying to create a simple reference class in R. Here is my code (R beginner):

MyClass <- setRefClass("MyClass",
                       fields = list(a = "numeric",
                                     b = "numeric"),

                       methods = list(
                         initialize <- function(){
                           print("Initializing")
                           a <<- 1
                           b <<- 2
                         },

                         printValues <- function(){
                           print(a)
                           print(b)
                         }
                         )
                       )

a <- MyClass$new()
a$printValues()

This produces the following error for the last line, a$printValues:

Error in envRefInferField(x, what, getClass(class(x)), selfEnv) : 
  "printValues" is not a valid field or method name for reference class “MyClass”

Also, the initializer method is not being called ?

Can someone point me to where the issue lies here ? Many thanks in advance.