What is Bone Heat weighting : Failed to find solution for one or more bones?

Hello guys,I’m trying to rigging for the first time but i got an error that said Bone Heat weighting : Failed to find solution for one or more bones,i already search it on google and watch a tutorial to fix that problem.But,no one of them work for me :no: .What i already do:
-remove doubles
-Recalculate Scale,Rotation,and Location
-applying all modifier before automatic weight

I don’t know what to do anymore :spin:.Is there anything i can do to fix that problem?Any help will be very appreciate (^^)

PS:Here the model: download (If you have spare times,please take a look at my model to see what is the problem thank you)