diff --git a/cpp/ConditionalGaussian.cpp b/cpp/ConditionalGaussian.cpp
index f456c68ef..f480a9874 100644
--- a/cpp/ConditionalGaussian.cpp
+++ b/cpp/ConditionalGaussian.cpp
@@ -87,10 +87,11 @@ bool ConditionalGaussian::equals(const ConditionalGaussian &cg, double tol) cons
 }
 
 /* ************************************************************************* */
-list<string> ConditionalGaussian::parents() {
+list<string> ConditionalGaussian::parents() const {
 	list<string> result;
 	for (Parents::const_iterator it = parents_.begin(); it != parents_.end(); it++)
 		result.push_back(it->first);
+	return result;
 }
 
 /* ************************************************************************* */
diff --git a/cpp/ConditionalGaussian.h b/cpp/ConditionalGaussian.h
index 9ba1d1ce9..2dbdacfe0 100644
--- a/cpp/ConditionalGaussian.h
+++ b/cpp/ConditionalGaussian.h
@@ -101,7 +101,7 @@ namespace gtsam {
     size_t dim() const {return R_.size2();}
 
     /** return all parents */
-    std::list<std::string> parents();
+    std::list<std::string> parents() const;
 
     /** return stuff contained in ConditionalGaussian */
     const Vector& get_d() const {return d_;}