diff --git a/src/time64.c b/src/time64.c
index 8c08caf..2ed8562 100644
--- a/src/time64.c
+++ b/src/time64.c
@@ -806,7 +806,9 @@
 char *ctime64_r( const Time64_T* time, char* result ) {
     struct TM date;
 
-    localtime64_r( time, &date );
+    if (!localtime64_r( time, &date ))
+        return NULL;
+
     return asctime64_r( &date, result );
 }