diff --git a/ml_date.cpp b/ml_date.cpp
index 71c906f..08d1e15 100644
--- a/ml_date.cpp
+++ b/ml_date.cpp
@@ -20,6 +20,7 @@ std::string date_to_string(const int datetime, const std::string format) {
 		return result;
 	}
 	// TODO exception here
+    return "invalid argument";
 }
 
 int string_to_date(const std::string &datestr, const std::string &format) {