diff --git a/gtsam/inference/BayesTree-inst.h b/gtsam/inference/BayesTree-inst.h
index b3fc8cb4c..9d631664d 100644
--- a/gtsam/inference/BayesTree-inst.h
+++ b/gtsam/inference/BayesTree-inst.h
@@ -321,60 +321,72 @@ namespace gtsam {
         ++p2;
       }
     }
-    if(!B)
-      throw std::invalid_argument("BayesTree::jointBayesNet does not yet work for joints across a forest");
     gttoc(Lowest_common_ancestor);
 
-    // Compute marginal on lowest common ancestor clique
-    gttic(LCA_marginal);
-    FactorGraphType p_B = B->marginal2(function);
-    gttoc(LCA_marginal);
-
-    // Compute shortcuts of the requested cliques given the lowest common ancestor
-    gttic(Clique_shortcuts);
-    BayesNetType p_C1_Bred = C1->shortcut(B, function);
-    BayesNetType p_C2_Bred = C2->shortcut(B, function);
-    gttoc(Clique_shortcuts);
-
-    // Factor the shortcuts to be conditioned on the full root
-    // Get the set of variables to eliminate, which is C1\B.
-    gttic(Full_root_factoring);
-    boost::shared_ptr<typename EliminationTraitsType::BayesTreeType> p_C1_B; {
-      FastVector<Index> C1_minus_B; {
-        FastSet<Index> C1_minus_B_set(C1->conditional()->beginParents(), C1->conditional()->endParents());
-        BOOST_FOREACH(const Index j, *B->conditional()) {
-          C1_minus_B_set.erase(j); }
-        C1_minus_B.assign(C1_minus_B_set.begin(), C1_minus_B_set.end());
-      }
-      // Factor into C1\B | B.
-      sharedFactorGraph temp_remaining;
-      boost::tie(p_C1_B, temp_remaining) =
-        FactorGraphType(p_C1_Bred).eliminatePartialMultifrontal(Ordering(C1_minus_B), function);
-    }
-    boost::shared_ptr<typename EliminationTraitsType::BayesTreeType> p_C2_B; {
-      FastVector<Index> C2_minus_B; {
-        FastSet<Index> C2_minus_B_set(C2->conditional()->beginParents(), C2->conditional()->endParents());
-        BOOST_FOREACH(const Index j, *B->conditional()) {
-          C2_minus_B_set.erase(j); }
-        C2_minus_B.assign(C2_minus_B_set.begin(), C2_minus_B_set.end());
-      }
-      // Factor into C2\B | B.
-      sharedFactorGraph temp_remaining;
-      boost::tie(p_C2_B, temp_remaining) =
-        FactorGraphType(p_C2_Bred).eliminatePartialMultifrontal(Ordering(C2_minus_B), function);
-    }
-    gttoc(Full_root_factoring);
-
-    gttic(Variable_joint);
     // Build joint on all involved variables
     FactorGraphType p_BC1C2;
-    p_BC1C2 += p_B;
-    p_BC1C2 += *p_C1_B;
-    p_BC1C2 += *p_C2_B;
-    if(C1 != B)
-      p_BC1C2 += C1->conditional();
-    if(C2 != B)
-      p_BC1C2 += C2->conditional();
+
+    if(B)
+    {
+      // Compute marginal on lowest common ancestor clique
+      gttic(LCA_marginal);
+      FactorGraphType p_B = B->marginal2(function);
+      gttoc(LCA_marginal);
+
+      // Compute shortcuts of the requested cliques given the lowest common ancestor
+      gttic(Clique_shortcuts);
+      BayesNetType p_C1_Bred = C1->shortcut(B, function);
+      BayesNetType p_C2_Bred = C2->shortcut(B, function);
+      gttoc(Clique_shortcuts);
+
+      // Factor the shortcuts to be conditioned on the full root
+      // Get the set of variables to eliminate, which is C1\B.
+      gttic(Full_root_factoring);
+      boost::shared_ptr<typename EliminationTraitsType::BayesTreeType> p_C1_B; {
+        FastVector<Index> C1_minus_B; {
+          FastSet<Index> C1_minus_B_set(C1->conditional()->beginParents(), C1->conditional()->endParents());
+          BOOST_FOREACH(const Index j, *B->conditional()) {
+            C1_minus_B_set.erase(j); }
+          C1_minus_B.assign(C1_minus_B_set.begin(), C1_minus_B_set.end());
+        }
+        // Factor into C1\B | B.
+        sharedFactorGraph temp_remaining;
+        boost::tie(p_C1_B, temp_remaining) =
+          FactorGraphType(p_C1_Bred).eliminatePartialMultifrontal(Ordering(C1_minus_B), function);
+      }
+      boost::shared_ptr<typename EliminationTraitsType::BayesTreeType> p_C2_B; {
+        FastVector<Index> C2_minus_B; {
+          FastSet<Index> C2_minus_B_set(C2->conditional()->beginParents(), C2->conditional()->endParents());
+          BOOST_FOREACH(const Index j, *B->conditional()) {
+            C2_minus_B_set.erase(j); }
+          C2_minus_B.assign(C2_minus_B_set.begin(), C2_minus_B_set.end());
+        }
+        // Factor into C2\B | B.
+        sharedFactorGraph temp_remaining;
+        boost::tie(p_C2_B, temp_remaining) =
+          FactorGraphType(p_C2_Bred).eliminatePartialMultifrontal(Ordering(C2_minus_B), function);
+      }
+      gttoc(Full_root_factoring);
+
+      gttic(Variable_joint);
+      p_BC1C2 += p_B;
+      p_BC1C2 += *p_C1_B;
+      p_BC1C2 += *p_C2_B;
+      if(C1 != B)
+        p_BC1C2 += C1->conditional();
+      if(C2 != B)
+        p_BC1C2 += C2->conditional();
+      gttoc(Variable_joint);
+    }
+    else
+    {
+      // The nodes have no common ancestor, they're in different trees, so they're joint is just the
+      // product of their marginals.
+      gttic(Disjoint_marginals);
+      p_BC1C2 += C1->marginal2(function);
+      p_BC1C2 += C2->marginal2(function);
+      gttoc(Disjoint_marginals);
+    }
 
     // now, marginalize out everything that is not variable j1 or j2
     return p_BC1C2.marginalMultifrontalBayesNet(Ordering(cref_list_of<2,Key>(j1)(j2)), boost::none, function);