diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 50ba456..5cde76c 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -142,7 +142,9 @@ jobs:
         run: |
           set -euo pipefail
           . .venv/bin/activate
-          PAIRS=$(python3 scripts/ci/distribute_molecule.py --runner-index ${{ matrix.runner-index }} --max-runners ${{ needs.discover-runners.outputs.runner-count }})
+          RUNNER_INDEX="${{ matrix.runner-index }}"
+          MAX_RUNNERS="${{ needs.discover-runners.outputs.runner-count }}"
+          PAIRS=$(python3 scripts/ci/distribute_molecule.py --runner-index "$RUNNER_INDEX" --max-runners "$MAX_RUNNERS")
           echo "Assigned pairs: $PAIRS"
           echo "TEST_PAIRS=$PAIRS" >> $GITHUB_ENV
       - name: Run molecule tests
diff --git a/scripts/ci/discover_runners.py b/scripts/ci/discover_runners.py
index bb63dce..8371b32 100644
--- a/scripts/ci/discover_runners.py
+++ b/scripts/ci/discover_runners.py
@@ -111,9 +111,14 @@ def get_runner_count(api_url: str, token: str, owner: str, repo: str) -> int:
     return DEFAULT_MAX_RUNNERS
 
 
-def generate_indices(count: int) -> list[int]:
-    """Generate a list of runner indices [0, 1, ..., count-1]."""
-    return list(range(count))
+def generate_indices(count: int) -> list[str]:
+    """Generate a list of runner indices ["0", "1", ..., "N-1"].
+
+    Uses strings instead of ints because Gitea Actions renders
+    integer 0 as empty in ${{ matrix.runner-index }} expressions,
+    causing --runner-index to be passed without a value.
+    """
+    return [str(i) for i in range(count)]
 
 
 @click.command()
diff --git a/tests/unit/test_discover_runners.py b/tests/unit/test_discover_runners.py
index 9cac15d..fbd3183 100644
--- a/tests/unit/test_discover_runners.py
+++ b/tests/unit/test_discover_runners.py
@@ -19,13 +19,13 @@ class TestGenerateIndices:
         assert generate_indices(0) == []
 
     def test_one(self) -> None:
-        assert generate_indices(1) == [0]
+        assert generate_indices(1) == ["0"]
 
     def test_three(self) -> None:
-        assert generate_indices(3) == [0, 1, 2]
+        assert generate_indices(3) == ["0", "1", "2"]
 
     def test_five(self) -> None:
-        assert generate_indices(5) == [0, 1, 2, 3, 4]
+        assert generate_indices(5) == ["0", "1", "2", "3", "4"]
 
 
 class TestQueryRunners:
@@ -166,7 +166,7 @@ class TestMain:
         result = runner.invoke(main, [])
         assert result.exit_code == 0
         assert "count=3" in result.output
-        assert "indices=[0, 1, 2]" in result.output
+        assert 'indices=["0", "1", "2"]' in result.output
 
     @patch("scripts.ci.discover_runners.get_runner_count", return_value=5)
     def test_count_only(self, mock_count: MagicMock) -> None:
@@ -180,11 +180,11 @@ class TestMain:
         runner = CliRunner()
         result = runner.invoke(main, ["--indices"])
         assert result.exit_code == 0
-        assert json.loads(result.output.strip()) == [0, 1, 2, 3]
+        assert json.loads(result.output.strip()) == ["0", "1", "2", "3"]
 
     @patch("scripts.ci.discover_runners.get_runner_count", return_value=1)
     def test_single_runner(self, mock_count: MagicMock) -> None:
         runner = CliRunner()
         result = runner.invoke(main, ["--indices"])
         assert result.exit_code == 0
-        assert json.loads(result.output.strip()) == [0]
+        assert json.loads(result.output.strip()) == ["0"]