I feel like this should be impossible as all my of dwellers are at least 75% happiness.

This happened when I was viewing my robots and explorer in the wasteland, as soon as I backed out of those, the report popped up with 0%.

http://i838.photobucket.com/albums/zz308/darkmaister/Screenshot_2015-10-01-09-45-40_zpsr5bys9vk.png