my code passed all of tests but when I tried to submit it, I got this error;

"NoMethodError: undefined method `*' for nil:NilClass"

My test numbers are below. What am I doing wrong?

Test.assert_equals(validate(79927398713), true)
Test.assert_equals(validate(79927398710), false)
Test.assert_equals(validate(79927398711), false)
Test.assert_equals(validate(79927398712), false)
Test.assert_equals(validate(79927398714), false)
Test.assert_equals(validate(2543), true)
Test.assert_equals(validate(2741), true)
Test.assert_equals(validate(2841), false)