diff --git a/lib/poper/runner.rb b/lib/poper/runner.rb
index 834c452..38fbcfa 100644
--- a/lib/poper/runner.rb
+++ b/lib/poper/runner.rb
@@ -32,7 +32,7 @@ def rules
     def commits
       @commits ||= begin
         walker.reset
-        walker.push(@repo.last_commit)
+        walker.push(@repo.last_commit.oid)
         walker.take_while { |c| c.oid != @commit.oid } << @commit
       end
     end