diff --git a/gtsam/navigation/expressions.h b/gtsam/navigation/expressions.h
index 231ccb20d..a6805deef 100644
--- a/gtsam/navigation/expressions.h
+++ b/gtsam/navigation/expressions.h
@@ -19,13 +19,13 @@ typedef Expression<Velocity3> Velocity3_;
 
 namespace internal {
 // define getters that return a value rather than a reference
-Rot3 attitude(const NavState& X, OptionalJacobian<3, 9> H) {
+inline Rot3 attitude(const NavState& X, OptionalJacobian<3, 9> H) {
   return X.attitude(H);
 }
-Point3 position(const NavState& X, OptionalJacobian<3, 9> H) {
+inline Point3 position(const NavState& X, OptionalJacobian<3, 9> H) {
   return X.position(H);
 }
-Velocity3 velocity(const NavState& X, OptionalJacobian<3, 9> H) {
+inline Velocity3 velocity(const NavState& X, OptionalJacobian<3, 9> H) {
   return X.velocity(H);
 }
 }  // namespace internal