diff --git a/ntt_parameters.h b/ntt_parameters.h
index c3da197..55671ec 100644
--- a/ntt_parameters.h
+++ b/ntt_parameters.h
@@ -168,7 +168,7 @@ struct NttParameters {
   ~NttParameters() = default;
 
   int number_coeffs;
-  std::optional<ModularInt> n_inv_ptr;
+  absl::optional<ModularInt> n_inv_ptr;
   std::vector<ModularInt> psis_bitrev;
   std::vector<ModularInt> psis_inv_bitrev;
   std::vector<unsigned int> bitrevs;