diff --git a/src/Makevars b/src/Makevars
index d715e49..812efd9 100755
--- a/src/Makevars
+++ b/src/Makevars
@@ -1,3 +1,3 @@
 CXX_STD = CXX11
-PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS)
+PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS) -DARMA_64BIT_WORD=1
 PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS) $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
diff --git a/src/Makevars.win b/src/Makevars.win
index d715e49..812efd9 100755
--- a/src/Makevars.win
+++ b/src/Makevars.win
@@ -1,3 +1,3 @@
 CXX_STD = CXX11
-PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS)
+PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS) -DARMA_64BIT_WORD=1
 PKG_LIBS = $(SHLIB_OPENMP_CXXFLAGS) $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)