diff --git a/src/modeci_mdf/functions/standard.py b/src/modeci_mdf/functions/standard.py
index af989d1a..9ae3809a 100644
--- a/src/modeci_mdf/functions/standard.py
+++ b/src/modeci_mdf/functions/standard.py
@@ -116,7 +116,11 @@ def substitute_args(expression_string: str = None, args: Dict[str, str] = None)
     return expression_string
 
 
-def create_python_function(name: str, expression_string: str, arguments: List[str]) -> types.FunctionType:
+def create_python_function(
+    name: str = None,
+    expression_string: str = None,
+    arguments: List[str] = None,
+) -> "types.FunctionType":
     """Create a Python function e.g. linear, exponential, sin, cos, ReLu
 
     Args:
@@ -133,7 +137,7 @@ def create_python_function(name: str, expression_string: str, arguments: List[st
     # assumes expression is one line
     name = name.replace(":", "_")
     expr = create_python_expression(expression_string)
-    func_str = f"def {name}({','.join(arguments)}):\n\treturn {expression_string}"
+    func_str = f"def {name}({','.join(arguments)}):\n\treturn {expr}"
 
     res = {}
     exec(func_str, globals(), res)