diff --git a/qsimcirq/qsim_circuit.py b/qsimcirq/qsim_circuit.py
index a5b4ae277..a78c5cae3 100644
--- a/qsimcirq/qsim_circuit.py
+++ b/qsimcirq/qsim_circuit.py
@@ -19,8 +19,8 @@
 
 from . import qsim
 
-# List of parameter names that appear in valid Cirq protos.
-GATE_PARAMS = [
+# Set of parameter names that appear in valid Cirq protos.
+GATE_PARAMS = {
     "exponent",
     "phase_exponent",
     "global_shift",
@@ -29,7 +29,7 @@
     "axis_phase_exponent",
     "phi",
     "theta",
-]
+}
 
 
 def _translate_ControlledGate(gate: cirq.ControlledGate):