diff --git a/HARK/core.py b/HARK/core.py
index dc316c0bb..0bc749908 100644
--- a/HARK/core.py
+++ b/HARK/core.py
@@ -57,7 +57,8 @@ def __init__(self, **parameters):
             Any number of keyword arguments of the form key=value.
             To parse a dictionary of parameters, use the ** operator.
         """
-        self._term_age = parameters.pop("T_cycle", None)
+        params = parameters.copy()
+        self._term_age = params.pop("T_cycle", None)
         self._age_inv = set()
         self._age_var = set()
         self._parameters = {}