diff --git a/src/share/ob_order_perserving_encoder.cpp b/src/share/ob_order_perserving_encoder.cpp
index 44f6f08c25..6969da2450 100644
--- a/src/share/ob_order_perserving_encoder.cpp
+++ b/src/share/ob_order_perserving_encoder.cpp
@@ -848,8 +848,15 @@ int ObSortkeyConditioner::process_key_conditioning(
     if (ret != OB_BUF_NOT_ENOUGH) {
       LOG_WARN("failed  to encode sortkey", K(ret));
     }
+  } else if (max_buf_len < to_len) {
+    ret = OB_BUF_NOT_ENOUGH;
+    LOG_TRACE("no enough memory to do encoding for obnumber", K(ret));
   } else if (!param.is_asc_) {
-    process_decrease(to + 1, to_len);
+    if (param.is_nullable_) {
+      process_decrease(to + 1, to_len - 1);
+    } else {
+      process_decrease(to, to_len);
+    }
   }
 
   return ret;