diff --git a/gtsam/hybrid/GaussianMixture.cpp b/gtsam/hybrid/GaussianMixture.cpp
index 5172a9798..fa0cc189b 100644
--- a/gtsam/hybrid/GaussianMixture.cpp
+++ b/gtsam/hybrid/GaussianMixture.cpp
@@ -129,7 +129,6 @@ void GaussianMixture::print(const std::string &s,
 }
 
 /* ************************************************************************* */
-/// Return the DiscreteKey vector as a set.
 std::set<DiscreteKey> DiscreteKeysAsSet(const DiscreteKeys &dkeys) {
   std::set<DiscreteKey> s;
   s.insert(dkeys.begin(), dkeys.end());
diff --git a/gtsam/hybrid/GaussianMixture.h b/gtsam/hybrid/GaussianMixture.h
index 9792a8532..7c1cde4cf 100644
--- a/gtsam/hybrid/GaussianMixture.h
+++ b/gtsam/hybrid/GaussianMixture.h
@@ -162,6 +162,9 @@ class GTSAM_EXPORT GaussianMixture
   Sum add(const Sum &sum) const;
 };
 
+/// Return the DiscreteKey vector as a set.
+std::set<DiscreteKey> DiscreteKeysAsSet(const DiscreteKeys &dkeys);
+
 // traits
 template <>
 struct traits<GaussianMixture> : public Testable<GaussianMixture> {};