How to extend PackageA's reference class in PackageB?

Question

The Problem

The diagram below shows my problem:

I am trying to create a reference class ClassB in PackageB which extends reference class ClassA in another package PackageA.

As soon as ClassB is instantiated, R complains that private (non-exported) objects (functions, in particular) of PackageA can't be found.

Minimal Working Example (MWE)

The diagram below shows how the MWE works:

ClassA depends on 2 private functions in PackageA (fractions from import-ed package MASS, and PrivateFunc defined within PackageA itself). ClassB implicitly also depends on these functions, through its inheritance of ClassA.

As soon as ClassB is instantiated, there is an error complaining that PrivateFunc cannot be found. These errors occur when I build PackageB.

The code:

##########################################
# PackageA

### R/PackageA.R ###
#' @import methods MASS
PrivateFunc <- function(...) fractions(0.5)   # try a function from MASS

#' @export ClassA
#' @exportClass ClassA
ClassA <- setRefClass("ClassA",
    methods = list(
        initialize = function(...) PrivateFunc()
    )
)

### DESCRIPTION: ###
Package: PackageA
...
Imports: methods, MASS

### NAMESPACE: ###
# Generated by roxygen2 (4.0.1): do not edit by hand
export(ClassA)
exportClasses(ClassA)
import(MASS)
import(methods)


##########################################
# PackageB

### R/PackageB.R ###
#' @import methods PackageA

#' @export ClassB
#' @exportClass ClassB
ClassB <- setRefClass("ClassB", contains = "ClassA")

#' @export ClassBInstance
ClassBInstance <- ClassB()     # <- Error here!!

### DESCRIPTION: ###
Package: PackageB
...
Imports: methods, PackageA


### NAMESPACE: ###
# Generated by roxygen2 (4.0.1): do not edit by hand
export(ClassB)
exportClasses(ClassB)
export(ClassBInstance)
import(PackageA)

The errors when I try to build PackageB:

==> devtools::document(roclets=c('rd', 'collate', 'namespace'))

Updating PackageB documentation
Loading PackageB
Error in .Object$initialize(...) : could not find function "PrivateFunc"
Error: Failure in roxygen block beginning PackageB.R:2
using 'as.environment(NULL)' is defunct
Execution halted

Exited with status 1.


==> Rcmd.exe INSTALL --no-multiarch --with-keep.source PackageB

* installing to library .../R/R-3.1.0/library'
* installing *source* package 'PackageB' ...
** R
** preparing package for lazy loading
Error in .Object$initialize(...) : could not find function "PrivateFunc"
Error : unable to load R code in package 'PackageB'
ERROR: lazy loading failed for package 'PackageB'
* removing '.../R/R-3.1.0/library/PackageB'
* restoring previous '.../R/R-3.1.0/library/PackageB'

Exited with status 1.

What's going wrong?

Additional Info

version
               _                           
platform       x86_64-w64-mingw32          
arch           x86_64                      
os             mingw32                     
system         x86_64, mingw32             
status                                     
major          3                           
minor          1.0                         
year           2014                        
month          04                          
day            10                          
svn rev        65387                       
language       R                           
version.string R version 3.1.0 (2014-04-10)
nickname       Spring Dance