Two tables - 'salaries' and 'master1'

The salaries is by year, and I can group them to get the sums for each player using 

```
SELECT playerID, sum(salary) as sal 
FROM salaries 
GROUP BY playerID ORDER BY sal DESC LIMIT 10; 

```

This returns the playerID and sum of salary, but I need the names of the players from the 'master1' table under column 'nameFirst' and 'nameLast'. They have the common column of 'playerID' in both 'master1' and 'salaries' but when I try to run

```
SELECT master1.nameFirst, master1.nameLast, sum(salary) as sal 
FROM salaries, master1 
GROUP BY salaries.playerID ORDER BY sal DESC LIMIT 10; 

```

I get the error
Expression not in `GROUP BY` key 'nameFirst'

I have tried tinkering with it to continue getting errors.

Thanks!