diff --git a/bigchaindb/consensus.py b/bigchaindb/consensus.py
index 818249b8..b65ab9ad 100644
--- a/bigchaindb/consensus.py
+++ b/bigchaindb/consensus.py
@@ -41,7 +41,7 @@ class AbstractConsensusRules(metaclass=ABCMeta):
             block (dict): block to validate.
 
         Returns:
-            The block if the block is valid else it raises and exception
+            The block if the block is valid else it raises an exception
             describing the reason why the block is invalid.
 
         Raises:
@@ -177,7 +177,7 @@ class BaseConsensusRules(AbstractConsensusRules):
             block (dict): block to validate.
 
         Returns:
-            The block if the block is valid else it raises and exception
+            The block if the block is valid else it raises an exception
             describing the reason why the block is invalid.
 
         Raises: