diff --git a/gusto/time_discretisation.py b/gusto/time_discretisation.py
index 2cdfb4b55..79c8af94e 100644
--- a/gusto/time_discretisation.py
+++ b/gusto/time_discretisation.py
@@ -95,7 +95,7 @@ def __init__(self, domain, field_name=None, solver_parameters=None,
             elif self.wrapper_name == "supg":
                 self.wrapper = SUPGWrapper(self, options)
             else:
-                raise NotImplementedError(
+                raise RuntimeError(
                     f'Time discretisation: wrapper {self.wrapper_name} not implemented')
         else:
             self.wrapper = None