[dba-SQLServer] executing system stored procedure

I've had this problem before but I don't recall the solution.

USE database
EXEC sys.sp_MSForEachTable command

says it can't find the stored procedure -- I remember I had this problem 
once before, but I don't remember why. Any help? When I run it against 
master, I get a command successfully completed, not a "can't find" message, 
so it must be there.

Susan H.