diff --git a/process/main.py b/process/main.py
index 0291d196..92834695 100644
--- a/process/main.py
+++ b/process/main.py
@@ -362,7 +362,7 @@ def run(self):
 class SingleRun:
     """Perform a single run of PROCESS."""
 
-    def __init__(self, input_file, update_obsolete, solver="vmcon"):
+    def __init__(self, input_file, update_obsolete=False, solver="vmcon"):
         """Read input file and initialise variables.
 
         :param input_file: input file named <optional_name>IN.DAT