"Error: package/namespace load failed"

I am writing my first R package, and I have been getting the following series of errors when I run R CMD check:

* checking S3 generic/method consistency ... WARNING
Error: package/namespace load failed for 'REEMtree'
Call sequence:
2: stop(gettextf("package/namespace load failed for '%s'", libraryPkgName(package)), 
       call. = FALSE, domain = NA)
1: library(package, lib.loc = lib.loc, character.only = TRUE, verbose = FALSE)
Execution halted
See section 'Generic functions and methods' of the 'Writing R Extensions'
manual.
* checking replacement functions ... WARNING
Error: package/namespace load failed for 'REEMtree'
Call sequence:
2: stop(gettextf("package/namespace load failed for '%s'", libraryPkgName(package)), 
       call. = FALSE, domain = NA)
1: library(package, lib.loc = lib.loc, character.only = TRUE, verbose = FALSE)
Execution halted
In R, the argument of a replacement function which corresponds to the right
hand side must be named 'value'.
* checking foreign function calls ... WARNING
Error: package/namespace load failed for 'REEMtree'
Call sequence:
2: stop(gettextf("package/namespace load failed for '%s'", libraryPkgName(package)), 
       call. = FALSE, domain = NA)
1: library(package, lib.loc = lib.loc, character.only = TRUE, verbose = FALSE)
Execution halted
See section 'System and foreign language interfaces' of the 'Writing R
Extensions' manual.
* checking Rd files ... OK
* checking for missing documentation entries ... ERROR
Error: package/namespace load failed for 'REEMtree'

(Everything is OK up to this point.)

Looking around online, I have found references to this error when there is compiled C or Fortran code, but I have none of that in my code.  I imagine this is a simple problem (perhaps with my NAMESPACE file), but I don't know what it is.  (The text of the NAMESPACE file is at the bottom of this e-mail.)

Thank you in advance for your help!

Rebecca

NAMESPACE file:

useDynLib(REEMtree)

export(AutoCorrelationLRtest, FixedEffectsTree, RandomEffectsTree, 
	LMEpredict, PredictionTest, RandomEffectsTree, RMSE, simpleREEMdata, 
	REEMtree, FEEMtree)

import(nlme)
import(rpart)

S3method(is,REEMtree)
S3method(logLik,REEMtree)
S3method(plot,REEMtree)
S3method(predict,REEMtree)
S3method(print, REEMtree)
S3method(ranef,REEMtree)
S3method(tree,REEMtree)
S3method(is,FEEMtree)
S3method(logLik,FEEMtree)
S3method(plot,FEEMtree)
S3method(predict,FEEMtree)
S3method(print, FEEMtree)
S3method(tree,FEEMtree)



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