Add epsilon argument check

pymc3/distributions/simulator.py

@@ -307,6 +307,13 @@ def check_simulator_args(cls, simulator, *args, **kwargs):
                 f"`pm.SimulatorRV` but got {type(simulator)}"
             )
 
+        n_params = len(args) + len(kwargs.get("params", [])) + ("epsilon" in kwargs)
+        if n_params != len(simulator.ndims_params):
+            raise ValueError(
+                f"`Simulator` expected {len(simulator.ndims_params)} parameters"
+                f"but got {n_params}. Did you forget to specify `epsilon`?"
+            )
+
         if "distance" in kwargs:
             raise ValueError(
                 "distance is no longer defined when calling `pm.Simulator`. It"

pymc3/tests/test_smc.py

@@ -445,6 +445,11 @@ def test_simulator_error_msg(self):
             with pytest.raises(ValueError, match=msg):
                 pm.Simulator("sim", lambda: None, 0, 1, epsilon=1.0)
 
+        msg = "Did you forget to specify `epsilon`?"
+        with pm.Model() as m:
+            with pytest.raises(ValueError, match=msg):
+                pm.Simulator("sim", NormalSimRV1(), 0, 1)
+
         msg = "distance is no longer defined when calling `pm.Simulator`"
         with pm.Model() as m:
             with pytest.raises(ValueError, match=msg):