diff --git a/include/finufft/finufft_core.h b/include/finufft/finufft_core.h
index a20106dc3..7e934addd 100644
--- a/include/finufft/finufft_core.h
+++ b/include/finufft/finufft_core.h
@@ -160,7 +160,7 @@ template<typename TF> struct FINUFFT_PLAN_T { // the main plan class, fully C++
 
   std::array<UBIGINT, 3> mstu; // number of modes in x/y/z dir (historical CMCL name) =
                                // N1/N2/N3
-  inline UBIGINT N() { return mstu[0]*mstu[1]*mstu[2]; } // total # modes (prod of above three)
+  inline UBIGINT N() const { return mstu[0]*mstu[1]*mstu[2]; } // total # modes (prod of above three)
 
   std::array<UBIGINT, 3> nf123{1, 1, 1}; // size of internal fine grid in x/y/z
                                          // direction