diff --git a/src/core/lib/flags.c b/src/core/lib/flags.c
index 527ea7f..3f0354b 100644
--- a/src/core/lib/flags.c
+++ b/src/core/lib/flags.c
@@ -24,7 +24,7 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 //CONSTS (this is just local)
 #define DEFAULT_FLAGS (why2_input_flags) { 0, 0, 0 }
-#define DEFAULT_MEMORY_IDENTIFIER NULL
+#define DEFAULT_MEMORY_IDENTIFIER ""
 
 int encryptionOperation(int text, int encryptedText);