Interface methods question

[sincnarf]

Here is an interface

#ifndef INTERFACES_H
#define INTERFACES_H
#include <QtPlugin>
#include <QtGui>
#include <QList>
class QImage;
class QString;
class QStringList;
 
class FilterInterface
{
public:
    virtual ~FilterInterface(){}
    virtual QStringList filters() const = 0;
 
	virtual QImage filterImage(const QString &filter, const QImage &image,
                                QWidget *parent) = 0;
	virtual QList<QImage> filterImageList(QString &filter, const QImage &image, QWidget *parent) = 0;
 
};
Q_DECLARE_INTERFACE(FilterInterface,
                     "com.iadfuq.sp.filterplugin.FilterInterface/1.0")
 
#endif

#ifndef INTERFACES_H
#define INTERFACES_H
#include <QtPlugin>
#include <QtGui>
#include <QList>
class QImage;
class QString;
class QStringList;

class FilterInterface
{
public:
    virtual ~FilterInterface(){}
    virtual QStringList filters() const = 0;
    
	virtual QImage filterImage(const QString &filter, const QImage &image,
                                QWidget *parent) = 0;
	virtual QList<QImage> filterImageList(QString &filter, const QImage &image, QWidget *parent) = 0;

};
Q_DECLARE_INTERFACE(FilterInterface,
                     "com.iadfuq.sp.filterplugin.FilterInterface/1.0")
                   
#endif

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This is copied from the plug and paint example of Qt. Only difference is I added a method called filterImageList that must return a QList of QImage.

Now I am trying to use with

const QList<QImage> images = iFilter->filterImageList(action->text(),
				pixmapItem->pixmap().toImage(), 0);

const QList<QImage> images = iFilter->filterImageList(action->text(),
				pixmapItem->pixmap().toImage(), 0);

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I get the error

mainwindow.cpp:1606: error: no matching function for call to ‘FilterInterface::filterImageList(QString, QImage, int)’
interfaces.h:20: note: candidates are: virtual QList<QImage> FilterInterface::filterImageList(QString&, const QImage&, QWidget*)

Is there something wrong with the way I declare QList<QImage> images?