--- sql/sql_table.cc.orig	2008-02-25 12:14:37.000000000 +0200
+++ sql/sql_table.cc	2008-02-25 12:14:41.000000000 +0200
@@ -5195,6 +5195,7 @@
       field->flags|= FIELD_IS_RENAMED;      
 
     /* Evaluate changes bitmap and send to check_if_incompatible_data() */
+    new_field->create_length_to_internal_length();
     if (!(tmp= field->is_equal(new_field)))
     {
       *need_copy_table= ALTER_TABLE_DATA_CHANGED;