diff --git a/cpp/LinearFactorGraph.cpp b/cpp/LinearFactorGraph.cpp
index f7733e772..b8c6aa630 100644
--- a/cpp/LinearFactorGraph.cpp
+++ b/cpp/LinearFactorGraph.cpp
@@ -155,7 +155,7 @@ FGConfig LinearFactorGraph::optimize(const Ordering& ordering)
 /* ************************************************************************* */
 /** combine two factor graphs                                                 */ 
 /* ************************************************************************* */
-void LinearFactorGraph::combine(LinearFactorGraph &lfg){
+void LinearFactorGraph::combine(const LinearFactorGraph &lfg){
 	for(const_iterator factor=lfg.factors.begin(); factor!=lfg.factors.end(); factor++){
 		push_back(*factor);
 	}
diff --git a/cpp/LinearFactorGraph.h b/cpp/LinearFactorGraph.h
index 04e3cd8bb..48a5414d5 100644
--- a/cpp/LinearFactorGraph.h
+++ b/cpp/LinearFactorGraph.h
@@ -105,7 +105,7 @@ namespace gtsam {
      * combine two factor graphs
      * @param *lfg Linear factor graph
      */
-    void combine(LinearFactorGraph &lfg);
+    void combine(const LinearFactorGraph &lfg);
 
     /**
      * Find all variables and their dimensions