diff --git a/sloth/annotations/container.py b/sloth/annotations/container.py
index c92fa1c..325b9e5 100644
--- a/sloth/annotations/container.py
+++ b/sloth/annotations/container.py
@@ -29,7 +29,7 @@ except:
         from PIL import Image
         _use_pil = True
     except:
-        LOG.warn("Could neither find PIL nor okapy.  Sloth needs one of them for loading images.")
+        raise RuntimeError("Could neither find PIL nor okapy.  Sloth needs one of them for loading images.")
 
 
 class AnnotationContainerFactory: