diff --git a/lib/resilient/circuit_breaker.rb b/lib/resilient/circuit_breaker.rb
index <HASH>..<HASH> 100644
--- a/lib/resilient/circuit_breaker.rb
+++ b/lib/resilient/circuit_breaker.rb
@@ -38,12 +38,14 @@ module Resilient
         else
           @metrics.mark_success
         end
+        nil
       }
     end
 
     def mark_failure
       instrument("resilient.circuit_breaker.mark_failure") { |payload|
         @metrics.mark_failure
+        nil
       }
     end
 
diff --git a/test/support/circuit_breaker_interface_test.rb b/test/support/circuit_breaker_interface_test.rb
index <HASH>..<HASH> 100644
--- a/test/support/circuit_breaker_interface_test.rb
+++ b/test/support/circuit_breaker_interface_test.rb
@@ -7,10 +7,18 @@ module CircuitBreakerInterfaceTest
     assert_respond_to @object, :mark_success
   end
 
+  def test_mark_success_returns_nothing
+    assert_nil @object.mark_success
+  end
+
   def test_responds_to_mark_failure
     assert_respond_to @object, :mark_failure
   end
 
+  def test_mark_failure_returns_nothing
+    assert_nil @object.mark_failure
+  end
+
   def test_responds_to_reset
     assert_respond_to @object, :reset
   end
Ensure circuit breaker mark success/failure return nothing