diff --git a/sql/sql_insert.cc b/sql/sql_insert.cc
index a38138ca5fd..9a7a3d64de5 100644
--- a/sql/sql_insert.cc
+++ b/sql/sql_insert.cc
@@ -103,7 +103,8 @@ static int check_insert_fields(THD *thd, TABLE_LIST *table_list,
         return -1;
     }
 #endif
-    *(int*)&table->timestamp_field_type&= ~ (int) TIMESTAMP_AUTO_SET_ON_INSERT;
+    clear_timestamp_auto_bits(table->timestamp_field_type,
+                              TIMESTAMP_AUTO_SET_ON_INSERT);
   }
   else
   {						// Part field list
@@ -150,7 +151,8 @@ static int check_insert_fields(THD *thd, TABLE_LIST *table_list,
     }
     if (table->timestamp_field &&	// Don't set timestamp if used
 	table->timestamp_field->query_id == thd->query_id)
-      *(int*)&table->timestamp_field_type&= ~ (int) TIMESTAMP_AUTO_SET_ON_INSERT;
+      clear_timestamp_auto_bits(table->timestamp_field_type,
+                                TIMESTAMP_AUTO_SET_ON_INSERT);
   }
   // For the values we need select_priv
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
@@ -216,7 +218,8 @@ static int check_update_fields(THD *thd, TABLE_LIST *insert_table_list,
   {
     /* Don't set timestamp column if this is modified. */
     if (table->timestamp_field->query_id == thd->query_id)
-      *(int*)&table->timestamp_field_type&= ~ (int) TIMESTAMP_AUTO_SET_ON_UPDATE;
+      clear_timestamp_auto_bits(table->timestamp_field_type,
+                                TIMESTAMP_AUTO_SET_ON_UPDATE);
     else
       table->timestamp_field->query_id= timestamp_query_id;
   }
diff --git a/sql/table.h b/sql/table.h
index d00ac41140e..0a4a06a75d1 100644
--- a/sql/table.h
+++ b/sql/table.h
@@ -71,18 +71,22 @@ typedef struct st_filesort_info
 
 
 /*
-  Values in this enum are used to indicate during which operations value
-  of TIMESTAMP field should be set to current timestamp.
-  WARNING: The values are used for bit operations. If you change the enum,
-  you must keep the bitwise relation of the values. For example:
-  (int) TIMESTAMP_AUTO_SET_ON_BOTH ==
-    (int) TIMESTAMP_AUTO_SET_ON_INSERT | (int) TIMESTAMP_AUTO_SET_ON_UPDATE.
+  Values in this enum are used to indicate how a tables TIMESTAMP field
+  should be treated. It can be set to the current timestamp on insert or
+  update or both.
+  WARNING: The values are used for bit operations. If you change the
+  enum, you must keep the bitwise relation of the values. For example:
+  (int) TIMESTAMP_AUTO_SET_ON_BOTH must be equal to
+  (int) TIMESTAMP_AUTO_SET_ON_INSERT | (int) TIMESTAMP_AUTO_SET_ON_UPDATE.
+  We use an enum here so that the debugger can display the value names.
 */
 enum timestamp_auto_set_type
 {
   TIMESTAMP_NO_AUTO_SET= 0, TIMESTAMP_AUTO_SET_ON_INSERT= 1,
   TIMESTAMP_AUTO_SET_ON_UPDATE= 2, TIMESTAMP_AUTO_SET_ON_BOTH= 3
 };
+#define clear_timestamp_auto_bits(_target_, _bits_) \
+  (_target_)= (enum timestamp_auto_set_type)((int)(_target_) & ~(int)(_bits_))
 
 class Field_timestamp;
 class Field_blob;