[Advaita-l] What is the Advaita message?

[Venkatesh Murthy (वेङ्कटेशः सीतारामार्यपुत्रः)]

Namaste

I do not understand what is the Advaita message in this Sloka -

गुणा गुणज्ञेषु गुणा भवन्ति ते निर्गुणं प्राप्य भवन्ति दोषाः ।
आस्वाद्यतोयाः प्रभवन्ति नद्यः समुद्रमासाद्य भवन्त्यपेयाः  ॥

Virtues become virtues in men understanding virtues. When they are in
a Nirguna person they become defects. The waters of rivers are tasty.
But when they reach the sea they become undrinkable.

Nirguna means person with no qualities or virtues? How can there be
virtues in a person with no virtues?

-- 
Regards

-Venkatesh