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If a given zipcode doesn't exist in the list of clients' addresses return "zipcode:/"

What happens if zipcode == ""?

Using C# I got...

Test Failed

Expected string length 2 but was 72. Strings differ at index 1.

Expected: ":/"

But was: ":,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,/,,,,,,,,,,,,,,,,,,,,,,,,..."
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But there's no where in the description that says under any condition to return ":/".