Impossible to create SIFT detector and extractor

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I'm trying to detect SIFT keypoints, using this code:

#include "opencv2/features2d/features2d.hpp"
#include "opencv2/nonfree/nonfree.hpp"
#include "opencv2/nonfree/features2d.hpp"

[...]

Ptr<FeatureDetector> detector = FeatureDetector::create( "SIFT" );
vector<KeyPoint> keyPoints;
detector->detect( image, keyPoints );

The problem is that my detector pointer points to nowhere after that last line, and no error is shown in the console.

I'm using QtCreator and my .pro file contains:

LIBS += -lopencv_nonfree
CONFIG += link_pkgconfig
PKGCONFIG += opencv

so my makefile contains:

LIBS          = $(SUBLIBS) -lopencv_nonfree [...]  /usr/local/lib/libopencv_nonfree.so [...]

There is no error when loading the libraries.

I've installed opencv using those instructions.