diff --git a/gtsam/nonlinear/GaussianISAM2-inl.h b/gtsam/nonlinear/GaussianISAM2-inl.h
index 822eb9674..b52e22629 100644
--- a/gtsam/nonlinear/GaussianISAM2-inl.h
+++ b/gtsam/nonlinear/GaussianISAM2-inl.h
@@ -109,7 +109,8 @@ namespace gtsam {
     vector<bool> changed(keys.size(), false);
     int count = 0;
     // starting from the root, call optimize on each conditional
-    internal::optimizeWildfire(root, threshold, changed, keys, delta, count);
+    if(root)
+      internal::optimizeWildfire(root, threshold, changed, keys, delta, count);
     return count;
   }