Qt/QSqlQuery: Error "No query Unable to fetch row"?

I am trying to execute the following code:

QString qryTxt = "INSERT INTO payments_" + getCurrentYear() + " (" + current_month + ") VALUES (" + QString::number(1) + ") WHERE ref_no = " + refNosList[i];
query.prepare(qryTxt);
if (!query.exec())
{
     qDebug() << "Error" << query.lastError().text();
     return false;
}

in the debugging mode this what I see as my qryTxt:

"INSERT INTO payments_2015 (January) VALUES (1) WHERE ref_no = 3"

But it is still returning the following error:

Error "No query Unable to fetch row"

P.S.: without the part saying WHERE ref_no = 3, the query works fine.

extra

• The ref_no (3) exists in the db
• table and column names are correct