diff --git a/bigchaindb/pipelines/block.py b/bigchaindb/pipelines/block.py
index b1d6cdee..fd503867 100644
--- a/bigchaindb/pipelines/block.py
+++ b/bigchaindb/pipelines/block.py
@@ -76,7 +76,7 @@ class BlockPipeline:
             tx.validate(self.bigchain)
             return tx
         except ValidationError as e:
-            logger.warning('Invalid tx: %s' % e)
+            logger.warning('Invalid tx: %s', e)
             self.bigchain.delete_transaction(tx.id)
             return None
 
diff --git a/bigchaindb/pipelines/vote.py b/bigchaindb/pipelines/vote.py
index 0431e20b..e4273470 100644
--- a/bigchaindb/pipelines/vote.py
+++ b/bigchaindb/pipelines/vote.py
@@ -109,7 +109,7 @@ class Vote:
             tx.validate(self.bigchain)
             valid = True
         except exceptions.ValidationError as e:
-            logger.warning('Invalid tx: %s' % e)
+            logger.warning('Invalid tx: %s', e)
             valid = False
 
         return valid, block_id, num_tx