diff --git a/.github/workflows/ci-daily.yml b/.github/workflows/ci-daily.yml
index d1f7606a5ed..4748c3d90c2 100644
--- a/.github/workflows/ci-daily.yml
+++ b/.github/workflows/ci-daily.yml
@@ -71,7 +71,8 @@ jobs:
     strategy:
       matrix:
         python-version: ['3.10', '3.11']
-    runs-on: macos-latest
+    # TODO(#6577): upgrade to macos-latest when it runs Python 3.10
+    runs-on: macos-13
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 13c7e397487..02a29248316 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -260,7 +260,8 @@ jobs:
     strategy:
       matrix:
         python-version: [ '3.10', '3.11' ]
-    runs-on: macos-latest
+    # TODO(#6577): upgrade to macos-latest when it runs Python 3.10
+    runs-on: macos-13
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
diff --git a/cirq-core/cirq/study/sweeps.py b/cirq-core/cirq/study/sweeps.py
index f075de48058..3f98798f602 100644
--- a/cirq-core/cirq/study/sweeps.py
+++ b/cirq-core/cirq/study/sweeps.py
@@ -236,8 +236,6 @@ def keys(self) -> List['cirq.TParamKey']:
         return sum((factor.keys for factor in self.factors), [])
 
     def __len__(self) -> int:
-        if not self.factors:
-            return 0
         length = 1
         for factor in self.factors:
             length *= len(factor)
diff --git a/cirq-core/cirq/study/sweeps_test.py b/cirq-core/cirq/study/sweeps_test.py
index aba82eacd23..83d0e0cc201 100644
--- a/cirq-core/cirq/study/sweeps_test.py
+++ b/cirq-core/cirq/study/sweeps_test.py
@@ -142,6 +142,11 @@ def test_product():
     assert _values(sweep, 'b') == [4, 5, 6, 7, 4, 5, 6, 7, 4, 5, 6, 7]
 
 
+def test_empty_product():
+    sweep = cirq.Product()
+    assert len(sweep) == len(list(sweep)) == 1
+
+
 def test_slice_access_error():
     sweep = cirq.Points('a', [1, 2, 3])
     with pytest.raises(TypeError, match='<class \'str\'>'):