Incompatible types

In the method addNoDuplicateSources, the if statement has an error message - incompatible types.
The input parameter to the method is a string and the return parameter from the getSource() method
is a String. Why would the if statement give an incompatible types message.?

Code Java:

import java.util.*;
public class AnotherWord
{
    private String word;
    private Vector<String> sourceList;
    private int count;
 
    public AnotherWord( String word, String source )
    {
        this.word = word.toLowerCase();
        count = 1;
        sourceList = new Vector<String> (3,2);
        sourceList.add( (String) source );
    }
 
    public void addNoDuplicateSources( String sourceToAdd)
    {
        boolean found = false;
        for (int i = 0; i <= sourceList.size(); i++)
        {
            if ( getSource(i).compareToIgnoreCase(sourceToAdd) ) { found = true; }
        }
        if (!found) { sourceList.add( sourceToAdd ); }
    }
 
    public String getSource(int index) 
    { 
        String result = null;
        if (index >=0 && index < sourceList.size()) { result = sourceList.get(index); }
        return result;
    }
}

--- Update ---

Ignore this post