diff --git a/include/openmc/cell.h b/include/openmc/cell.h
index d3b411dda..3df2f40d5 100644
--- a/include/openmc/cell.h
+++ b/include/openmc/cell.h
@@ -300,28 +300,18 @@ struct CellInstance {
   gsl::index index_cell;
   gsl::index instance;
 };
-} // namespace openmc
 
-namespace std {
-template<>
-struct hash<openmc::CellInstance> {
-  // Taken from https://stackoverflow.com/a/17017281
-  std::size_t operator()(const openmc::CellInstance& k) const
+struct CellInstanceHash {
+  std::size_t operator()(const CellInstance& k) const
   {
-    std::size_t res = 17;
-    res = 31 * res + std::hash<gsl::index>()(k.index_cell);
-    res = 31 * res + std::hash<gsl::index>()(k.instance);
-    return res;
+    return 4096*k.index_cell + k.instance;
   }
 };
-} // namespace std
 
 //==============================================================================
 // Non-member functions
 //==============================================================================
 
-namespace openmc {
-
 void read_cells(pugi::xml_node node);
 
 #ifdef DAGMC
diff --git a/include/openmc/tallies/filter_cell_instance.h b/include/openmc/tallies/filter_cell_instance.h
index 33874b85d..1b2cdd425 100644
--- a/include/openmc/tallies/filter_cell_instance.h
+++ b/include/openmc/tallies/filter_cell_instance.h
@@ -55,7 +55,7 @@ private:
   std::vector<CellInstance> cell_instances_;
 
   //! A map from cell/instance indices to filter bin indices.
-  std::unordered_map<CellInstance, gsl::index> map_;
+  std::unordered_map<CellInstance, gsl::index, CellInstanceHash> map_;
 };
 
 } // namespace openmc