diff --git a/cirq-core/cirq/experiments/single_qubit_readout_calibration.py b/cirq-core/cirq/experiments/single_qubit_readout_calibration.py
index 882d1fcee34..003ab34f365 100644
--- a/cirq-core/cirq/experiments/single_qubit_readout_calibration.py
+++ b/cirq-core/cirq/experiments/single_qubit_readout_calibration.py
@@ -70,11 +70,18 @@ def plot_heatmap(
             **plot_kwargs: Arguments to be passed to 'cirq.Heatmap.plot()'.
         Returns:
             The two plt.Axes containing the plot.
+
+        Raises:
+            ValueError if axs does not contain two plt.Axes
+            TypeError if qubits are not cirq.GridQubits
         """
 
         if axs is None:
             _, axs = plt.subplots(1, 2, dpi=200, facecolor='white', figsize=(12, 4))
 
+        else:
+            if not len(axes) != 2 or type(axs[0]) != plt.Axes or type(axs[1]) != plt.Axes:
+                raise ValueError('axs should be a length-2 tuple of plt.Axes')
         for ax, title, data in zip(
             axs,
             ['$|0\\rangle$ errors', '$|1\\rangle$ errors'],
@@ -82,7 +89,9 @@ def plot_heatmap(
         ):
             data_with_grid_qubit_keys = {}
             for qubit in data:
-                assert type(qubit) == grid_qubit.GridQubit, "qubits must be cirq.GridQubits"
+                if type(qubit) != grid_qubit.GridQubit:
+                    raise TypeError(f'{qubit} must be of type cirq.GridQubit')
+                cast(grid_qubit.GridQubit, qubit)
                 data_with_grid_qubit_keys[qubit] = data[qubit]  # just for typecheck
             _, _ = cirq_heatmap.Heatmap(data_with_grid_qubit_keys).plot(
                 ax, annotation_format=annotation_format, title=title, **plot_kwargs