diff --git a/gtsam/base/Manifold.h b/gtsam/base/Manifold.h
index b07bd3575..ba26d267d 100644
--- a/gtsam/base/Manifold.h
+++ b/gtsam/base/Manifold.h
@@ -81,7 +81,7 @@ struct ManifoldImpl {
 
 /// Extra manifold traits for variable-dimension types
 template<class Class>
-struct ManifoldImpl<Class, Eigen::Dynamic> {
+    struct ManifoldImpl<Class, size_t(Eigen::Dynamic)> {
   // Run-time dimensionality
   static int GetDimension(const Class& m) {
     return m.dim();