Replacing equal sign with colon alone does not solve the problem. It changes the problem by making the value for option abc optional instead of being required; equals sign impose that a value need to be specified ...

  = type [ desttype ] [ repeat ]
    The option requires an argument of the given type.

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  : type [ desttype ]
    Like "=", but designates the argument as optional.  If
    omitted, an empty string will be assigned to string
    values options, and the value zero to numeric options.
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