diff --git a/python/simulation.py b/python/simulation.py
index 60538271d..3bf01a092 100644
--- a/python/simulation.py
+++ b/python/simulation.py
@@ -3,6 +3,7 @@
 """
 
 import functools
+import inspect
 import math
 import numbers
 import os
@@ -93,11 +94,12 @@ def fix_dft_args(args, i):
 
 
 def get_num_args(func):
-    return (
-        2
-        if isinstance(func, Harminv) or isinstance(func, PadeDFT)
-        else func.__code__.co_argcount
-    )
+    if isinstance(func, Harminv) or isinstance(func, PadeDFT):
+        return 2
+    elif inspect.ismethod(func):
+        return func.__code__.co_argcount - 1  # remove 'self' from count
+    else:
+        return func.__code__.co_argcount
 
 
 def vec(*args):