diff --git a/daliuge-engine/dlg/drop.py b/daliuge-engine/dlg/drop.py
index 4fd5c1a09..70cdd2614 100644
--- a/daliuge-engine/dlg/drop.py
+++ b/daliuge-engine/dlg/drop.py
@@ -468,7 +468,7 @@ def _popArg(self, kwargs, key, default):
         Pops the specified key arg from kwargs else returns the default
         """
         if key not in kwargs:
-            logger.debug("Defaulting %s to %s in %r" % (key, str(default), self))
+            logger.debug("Defaulting %s to %s in %r", key, str(default), self)
         return kwargs.pop(key, default)
 
     def __hash__(self):