i >= 0
If you're getting Assertion, it means that the expression evaluates to false, therefore "i" has to be less than zero (probably error, as -1 is often used by the devs to indicate error).
Asserts are only available in Debug mode builds, or when explicitly enabled in Release mode.
I'd say that you should look for entries with no values, maybe you've left some entry somewhere with no value assigned ? Just a guess 
Cheers
PS> Really, only devs know exactly what is causing this, but your report tells exactly where it fails.