diff --git a/keras_rcnn/preprocessing/_object_detection.py b/keras_rcnn/preprocessing/_object_detection.py
index c6dda5de..f62ffff0 100644
--- a/keras_rcnn/preprocessing/_object_detection.py
+++ b/keras_rcnn/preprocessing/_object_detection.py
@@ -126,6 +126,13 @@ def next(self):
                         label[self.classes[b["class"]]] = 1
                         labels = numpy.append(labels, [[label]], axis=1)
 
+                # check of all boxes are valid, i.e. x1 < x2 and y1 < y2
+                for i, b in enumerate(self.dictionary[image_index]["boxes"]):
+                        if ( b["x1"] >=  b["x2"] ) or ( b["x1"] >=  b["x2"] ):
+                            raise Exception(
+                                "invalid bounding boxes (x1 > x2 or y1 > y2"
+                            )
+
             # Scale the ground truth boxes to the selected image scale.
             boxes[batch_index, :, :4] *= self.scale