Kelis -- Her Milkshake Brings All the TAX LIENS

Kelis apparently took away one very important life lesson from her ex-hubby Nas ... mainly how NOT to pay her taxes ... to the tune of $300,000.

According to a tax lien filed in NYC, the government wants Kelis to fork up over $330,426.71 in unpaid taxes for the years 2004 and 2010.

Both 2004 & 2010 were good years for Kelis. In 2004, the "Milkshake" singer went on tour with Britney Spears. In 2010, she headlined her first US tour with singer Robyn.