diff --git a/include/boost/qvm/mat.hpp b/include/boost/qvm/mat.hpp
index 9fc118d..889c263 100644
--- a/include/boost/qvm/mat.hpp
+++ b/include/boost/qvm/mat.hpp
@@ -21,7 +21,7 @@ mat
         , class = typename enable_if<is_mat<R> >::type
 #endif
     >
-    operator R() const
+    BOOST_QVM_CONSTEXPR operator R() const
         {
         R r;
         assign(r,*this);
diff --git a/include/boost/qvm/quat.hpp b/include/boost/qvm/quat.hpp
index 867ff69..4ee4069 100644
--- a/include/boost/qvm/quat.hpp
+++ b/include/boost/qvm/quat.hpp
@@ -21,7 +21,7 @@ quat
         , class = typename enable_if<is_quat<R> >::type
 #endif
     >
-    operator R() const
+    BOOST_QVM_CONSTEXPR operator R() const
         {
         R r;
         assign(r,*this);
diff --git a/include/boost/qvm/vec.hpp b/include/boost/qvm/vec.hpp
index 6e5cb53..8adc003 100644
--- a/include/boost/qvm/vec.hpp
+++ b/include/boost/qvm/vec.hpp
@@ -21,7 +21,7 @@ vec
         , class = typename enable_if<is_vec<R> >::type
 #endif
     >
-    operator R() const
+    BOOST_QVM_CONSTEXPR operator R() const
         {
         R r;
         assign(r,*this);