diff --git a/byterun/pyvm2.py b/byterun/pyvm2.py
index 394e2e0b..2855df88 100644
--- a/byterun/pyvm2.py
+++ b/byterun/pyvm2.py
@@ -950,7 +950,7 @@ def call_function(self, arg, args, kwargs):
         frame = self.frame
         if hasattr(func, 'im_func'):
             # Methods get self as an implicit first parameter.
-            if func.im_self:
+            if func.im_self is not None:
                 posargs.insert(0, func.im_self)
             # The first parameter must be the correct type.
             if not isinstance(posargs[0], func.im_class):