diff --git a/cirq-core/cirq/testing/consistent_channels.py b/cirq-core/cirq/testing/consistent_channels.py
index a9e65384dc8..1b7029955fd 100644
--- a/cirq-core/cirq/testing/consistent_channels.py
+++ b/cirq-core/cirq/testing/consistent_channels.py
@@ -33,8 +33,8 @@ def assert_consistent_mixture(gate: Any, rtol: float = 1e-5, atol: float = 1e-8)
     """Asserts that a given gate is a mixture and the mixture probabilities sum to one."""
     assert cirq.has_mixture(gate), f"Give gate {gate!r} does not return for cirq.has_mixture."
     mixture = cirq.mixture(gate)
-    total = np.sum(k for k, v in mixture)
-    assert total - 1 <= atol + rtol * np.abs(total), (
+    total = np.sum(np.fromiter((k for k, v in mixture), dtype=float))
+    assert np.abs(1 - total) <= atol + rtol * np.abs(total), (
         f"The mixture for gate {gate!r} did not return coefficients that sum to 1. Summed to "
         f"{total}."
     )
diff --git a/cirq-core/cirq/testing/consistent_channels_test.py b/cirq-core/cirq/testing/consistent_channels_test.py
index 57662d79f72..75ef7770653 100644
--- a/cirq-core/cirq/testing/consistent_channels_test.py
+++ b/cirq-core/cirq/testing/consistent_channels_test.py
@@ -45,3 +45,48 @@ def test_assert_consistent_channel_invalid():
 def test_assert_consistent_channel_not_kraus():
     with pytest.raises(AssertionError, match="12.*has_kraus"):
         cirq.testing.assert_consistent_channel(12)
+
+
+def test_assert_consistent_mixture_valid():
+    mixture = cirq.X.with_probability(0.1)
+    cirq.testing.assert_consistent_mixture(mixture)
+
+
+def test_assert_consistent_mixture_not_mixture():
+    not_mixture = cirq.amplitude_damp(0.1)
+    with pytest.raises(AssertionError, match="has_mixture"):
+        cirq.testing.assert_consistent_mixture(not_mixture)
+
+
+class _MixtureGate(cirq.testing.SingleQubitGate):
+    def __init__(self, p, q):
+        self._p = p
+        self._q = q
+        super().__init__()
+
+    def _mixture_(self):
+        return (self._p, cirq.unitary(cirq.I)), (self._q, cirq.unitary(cirq.X))
+
+
+def test_assert_consistent_mixture_not_normalized():
+    mixture = _MixtureGate(0.1, 0.85)
+    with pytest.raises(AssertionError, match="sum to 1"):
+        cirq.testing.assert_consistent_mixture(mixture)
+
+    mixture = _MixtureGate(0.2, 0.85)
+    with pytest.raises(AssertionError, match="sum to 1"):
+        cirq.testing.assert_consistent_mixture(mixture)
+
+
+def test_assert_consistent_mixture_tolerances():
+
+    # This gate is 1e-5 off being properly normalized.
+    mixture = _MixtureGate(0.1, 0.9 - 1e-5)
+    # Defaults of rtol=1e-5, atol=1e-8 are fine.
+    cirq.testing.assert_consistent_mixture(mixture)
+
+    with pytest.raises(AssertionError, match="sum to 1"):
+        cirq.testing.assert_consistent_mixture(mixture, rtol=0, atol=1e-6)
+
+    with pytest.raises(AssertionError, match="sum to 1"):
+        cirq.testing.assert_consistent_mixture(mixture, rtol=1e-6, atol=0)