diff --git a/cpp/NonlinearOptimizer-inl.h b/cpp/NonlinearOptimizer-inl.h
index e76c104cf..844386fe9 100644
--- a/cpp/NonlinearOptimizer-inl.h
+++ b/cpp/NonlinearOptimizer-inl.h
@@ -18,7 +18,7 @@ using namespace std;
 namespace gtsam {
 
 	/* ************************************************************************* */
-	bool check_convergence(double relativeErrorTreshold,
+	inline bool check_convergence(double relativeErrorTreshold,
 			double absoluteErrorTreshold, double currentError, double newError,
 			int verbosity) {
 		// check if diverges