diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index b79ea7d..abe6e88 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -7,7 +7,7 @@ jobs:
     name: build (${{ matrix.ruby }} / ${{ matrix.os }})
     strategy:
       matrix:
-        ruby: [ '3.0', 2.7, 2.6, head ]
+        ruby: [ '3.0', 2.7, 2.6, 2.5, head ]
         os: [ ubuntu-latest, macos-latest, windows-latest ]
     runs-on: ${{ matrix.os }}
     steps:
diff --git a/lib/ruby2_keywords.rb b/lib/ruby2_keywords.rb
index 97cd081..7a3f2fa 100644
--- a/lib/ruby2_keywords.rb
+++ b/lib/ruby2_keywords.rb
@@ -1,5 +1,5 @@
 class Module
-  unless private_method_defined?(:ruby2_keywords, true)
+  unless private_method_defined?(:ruby2_keywords)
     private
     # call-seq:
     #    ruby2_keywords(method_name, ...)