diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 919e5c8f..ea78321f 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -7,11 +7,29 @@ on:
     branches: ["*"]
 
 jobs:
+  determine_version:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+    steps:
+      - id: set-matrix
+        run: |
+          if [[ "${{ github.event_name }}" == 'push' && "${{ github.ref }}" == 'refs/heads/master' ]]; then
+            # Push to master branch
+            echo 'matrix=["3.7", "3.9", "3.10", "3.11"]' >> $GITHUB_OUTPUT
+          elif [[ "${{ github.event_name }}" == 'pull_request' && "${{ github.event.pull_request.base.ref }}" == 'master' ]]; then
+            # PR to master branch
+            echo 'matrix=["3.7", "3.9", "3.10", "3.11"]' >> $GITHUB_OUTPUT
+          else
+            echo 'matrix=["3.9"]' >> $GITHUB_OUTPUT
+          fi
+
   build:
+    needs: determine_version
     strategy:
       matrix:
         platform: [ubuntu-latest]
-        python-version: [3.7, 3.9]
+        python-version: ${{fromJson(needs.determine_version.outputs.matrix)}}
 
     runs-on: ${{ matrix.platform }}
 
diff --git a/hed/models/base_input.py b/hed/models/base_input.py
index 9f437102..58bb94c9 100644
--- a/hed/models/base_input.py
+++ b/hed/models/base_input.py
@@ -137,7 +137,7 @@ def _indexed_dict_from_onsets(onsets):
 
     @staticmethod
     def _filter_by_index_list(original_series, indexed_dict):
-        new_series = pd.Series(["n/a"] * len(original_series))
+        new_series = pd.Series(["n/a"] * len(original_series), dtype=str)
 
         for onset, indices in indexed_dict.items():
             if indices:
diff --git a/tests/models/test_base_input.py b/tests/models/test_base_input.py
index 71e21386..02c7f34a 100644
--- a/tests/models/test_base_input.py
+++ b/tests/models/test_base_input.py
@@ -304,7 +304,7 @@ def test_complex_onsets(self):
                          {3.5: [0, 1], 4.0: [2], 4.4: [3, 4], -1.0: [5]})
 
     def test_empty_and_single_item_series(self):
-        self.assertTrue(BaseInput._filter_by_index_list(pd.Series([]), {}).equals(pd.Series([])))
+        self.assertTrue(BaseInput._filter_by_index_list(pd.Series([], dtype=str), {}).equals(pd.Series([], dtype=str)))
         self.assertTrue(BaseInput._filter_by_index_list(pd.Series(["apple"]), {0: [0]}).equals(pd.Series(["apple"])))
 
     def test_two_item_series_with_same_onset(self):