diff --git a/include/boost/unordered/detail/foa.hpp b/include/boost/unordered/detail/foa.hpp
index 6bcc2b45..2970b594 100644
--- a/include/boost/unordered/detail/foa.hpp
+++ b/include/boost/unordered/detail/foa.hpp
@@ -680,7 +680,7 @@ private:
       new_arrays_.groups=boost::to_address(group_alloc_traits::allocate(gal,groups_size));
       // TODO: explain why memset
       std::memset(
-        new_arrays_.groups,0,sizeof(group_type)*groups_size-1);
+        new_arrays_.groups,0,sizeof(group_type)*groups_size);
       new_arrays_.groups[groups_size-1].set_sentinel();
       BOOST_TRY{
         new_arrays_.elements=boost::to_address(alloc_traits::allocate(al,groups_size*N-1));