diff --git a/libs/libc/time/lib_asctimer.c b/libs/libc/time/lib_asctimer.c
index 3f32301ef95..14ccf2b206c 100644
--- a/libs/libc/time/lib_asctimer.c
+++ b/libs/libc/time/lib_asctimer.c
@@ -28,6 +28,14 @@
 
 #include <nuttx/time.h>
 
+/****************************************************************************
+ * Pre-processor definitions
+ ****************************************************************************/
+
+#ifndef ARRAY_SIZE
+#  define ARRAY_SIZE(x)         (sizeof(x) / sizeof((x)[0]))
+#endif
+
 /****************************************************************************
  * Private Data
  ****************************************************************************/
@@ -73,6 +81,13 @@ FAR char *asctime_r(FAR const struct tm *tp, FAR char *buf)
 {
   char tmp[128];
 
+  if (tp == NULL ||
+      tp->tm_wday >= ARRAY_SIZE(g_wday_name) ||
+      tp->tm_mon >= ARRAY_SIZE(g_mon_name))
+    {
+      return NULL;
+    }
+
   snprintf(tmp, sizeof(tmp), "%.3s %.3s%3d %.2d:%.2d:%.2d %d\n",
            g_wday_name[tp->tm_wday], g_mon_name[tp->tm_mon],
            tp->tm_mday, tp->tm_hour, tp->tm_min, tp->tm_sec,