diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 006c9d9..6262c96 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -18,9 +18,11 @@ jobs:
   test:
     runs-on: ubuntu-latest
 
-    # test with multiple versions of ruby, must include the version in .ruby-version
     strategy:
+      # one at a time because concurrent runs invalidate each other
+      max-parallel: 1
       matrix:
+        # test with multiple versions of ruby, must include the version in .ruby-version
         ruby-version: ['2.5.9', '2.7.8']
 
     steps:
diff --git a/Gemfile.lock b/Gemfile.lock
index 52a0cde..cd1f6ab 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -92,4 +92,4 @@ DEPENDENCIES
   yard
 
 BUNDLED WITH
-   1.17.3
+   2.2.26