diff --git a/gtsam/slam/PriorFactor.h b/gtsam/slam/PriorFactor.h
index bc5e9c87f..7f7aef8cb 100644
--- a/gtsam/slam/PriorFactor.h
+++ b/gtsam/slam/PriorFactor.h
@@ -81,7 +81,7 @@ namespace gtsam {
 
     /** vector of errors */
     Vector evaluateError(const T& p, boost::optional<Matrix&> H = boost::none) const {
-      if (H) (*H) = eye(p.dim());
+      if (H) (*H) = eye(traits::dimension<T>());
       // manifold equivalent of h(x)-z -> log(z,h(x))
       DefaultChart<T> chart;
       return chart.local(prior_,p);