I am getting an error with the line "mail.setToAddresses" in the following code:

String email = '';
String ownerName = '';
// Check if Owner is a User
if (ownerType == 'User'){
	User owner = [select Name, Email from User where Id = :c.OwnerId];
	email = owner.Email;
	ownerName = owner.Name;
	System.debug('Email: ' + email + ', Name: ' + ownerName);
	String[] toAddresses = new String[]{email};
	mail.setToAddresses(toAddresses);
        ...

 The debug message is outputting the Email (and Name) just fine, so I am not sure what the problem is. I use the same syntax is another method and it works just fine, so I'm really not sure what the issue is. Any ideas?