diff --git a/include/my_bitmap.h b/include/my_bitmap.h
index 5589236664d..42a093c5bed 100644
--- a/include/my_bitmap.h
+++ b/include/my_bitmap.h
@@ -88,6 +88,7 @@ extern void bitmap_lock_clear_bit(MY_BITMAP *map, uint bitmap_bit);
 
 #define my_bitmap_map_bytes sizeof(my_bitmap_map)
 #define my_bitmap_map_bits  (my_bitmap_map_bytes*8)
+/* Size in bytes to store 'bits' number of bits */
 #define bitmap_buffer_size(bits) (MY_ALIGN((bits), my_bitmap_map_bits)/8)
 #define my_bitmap_buffer_size(map) bitmap_buffer_size((map)->n_bits)
 #define no_bytes_in_export_map(map) (((map)->n_bits + 7)/8)
diff --git a/sql/sql_update.cc b/sql/sql_update.cc
index 81910cf651b..8617c41c522 100644
--- a/sql/sql_update.cc
+++ b/sql/sql_update.cc
@@ -257,7 +257,7 @@ static void prepare_record_for_error_message(int error, TABLE *table)
   Field *field;
   uint keynr;
   MY_BITMAP unique_map; /* Fields in offended unique. */
-  my_bitmap_map unique_map_buf[bitmap_buffer_size(MAX_FIELDS)];
+  my_bitmap_map unique_map_buf[bitmap_buffer_size(MAX_FIELDS)/sizeof(my_bitmap_map)];
   DBUG_ENTER("prepare_record_for_error_message");
 
   /*