End Mill Without Flat Spot

I bought an 1/8" carbide tipped end mill recently for a project that became unneeded.   I bought an 1/8th" shank end mill holder from Sherline.  When I prepared to install it, I discovered that there was no flat onto which the set screw locks.  The holder had two setscrews so I was able to get an adequate grab to use it for cutting acetal.  It just seemed odd.  Have you ever seen an end mill without a flat?