crack in wall above fireplace

our fireplace has a brick front extending about 18 inches above fireplace. there's a vertical crack running up from firebox to mantle. this crack is barely noticable when fireplace is not in use, but when using, the crack widens to about an inch wide. you can look through the crack and see the fire.
i know this is dangerous.. we don't use the fireplace.. but now we are selling the house and i want to repair it. all mason's i speak to don't seem to believe me when i explain this to them.
what could it be? someone told me prehaps the lentil doesn't have enough room to expand when hot and it's pushing the sidewalls out.
how could it be fixed? slightly smaller lentil?
thanks for any advice...
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