diff --git a/.github/workflows/notebook_colab_gen.yml b/.github/workflows/notebook_colab_gen.yml
index e465bb5..bfbcf57 100644
--- a/.github/workflows/notebook_colab_gen.yml
+++ b/.github/workflows/notebook_colab_gen.yml
@@ -4,7 +4,7 @@ on: [push]
 env:
   notebook_dir: "./notebooks"
   colab_directory: "./colab_notebooks"
-  branch: $GITHUB_HEAD_REF
+  branch: ${{ GITHUB_HEAD_REF }}
 jobs:
   add_nbs:
     runs-on: ubuntu-latest