Because of changes in R, -DNDEBUG is automatic.

dmbates · dmbates · commit 0fa85aeb4ff1 · 2012-07-28T18:18:59.000Z
One must override it with -UNDEBUG in the local ~/.R/Makevars to
activate the debugging code.
diff --git a/src/Makevars b/src/Makevars
@@ -1,10 +1,5 @@
 ## -*- mode: makefile; -*-
 
-PKG_CXXFLAGS=-I../inst/include -DNDEBUG
-
-## In practice the compiler flags should be
-##PKG_CXXFLAGS=-I../inst/include
-## to enable error checking.  However, checking for errors is flagged
-## in R CMD check as an undesirable practice.
+PKG_CXXFLAGS=-I../inst/include
 
 PKG_LIBS=`$(R_HOME)/bin/Rscript -e "Rcpp:::LdFlags()"` $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
