error message when using html format for text

Hello ,
When I want to apply various size in a text sequence using html format, I've got this error code :

TypeError: Error #1034: Echec de la contrainte de type : conversion de flash.text::TextField@2c660ad9 en flash.display.MovieClip impossible.

Does somebody can help me to solve this prob ?

Please give me more details and tell what you did on each step. Thank you.