Unknown error: cannot claim child & dependent care credit because "you can be claimed as a dependent on someone else's return"

I encountered a strange denial message when going through the Child and Dependent Care Credit.
One of my dependent boys has child care (age 12) and meets all criteria  (expenses paid, valid EIN number, etc).   I had minor expenses (after you subtract the FSA child-care amount).  I don't expect much credit or maybe zero, but when I reviewed the Child and Dependent Care Credit section -  It asks "Do you want to work on the child and dep. care credit now?" -  Yes
  Then it says "It turns out you won't be able to claim the child and dependent care credit using TurboTax because you can be claimed as a dependent on someone else's return."   Ha ha!  Now I'm curious - who else can claim me?  
So - what kind of entry error could have led to this message?  I'm filing as head of household.  I don't mind not getting the child care credit, but I'd like to get an accurate explanation why not.