diff --git a/datasets/coco.py b/datasets/coco.py
index c006f44..a25fa14 100644
--- a/datasets/coco.py
+++ b/datasets/coco.py
@@ -120,7 +120,7 @@ def image_aspect_ratio(self, image_index):
         return float(image['width']) / float(image['height'])
 
     def num_classes(self):
-        return 80
+        return len(self.coco.getCatIds())
 
 
 if __name__ == '__main__':