The message suggests that you are calling the distance() function with only one variable; or with (at least one) of the variables containing undef.
Look to your own code where you call the function and print out the value(s) you are passing.
With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.