diff --git a/module/interface.py b/module/interface.py
index 544202ac..4b837886 100644
--- a/module/interface.py
+++ b/module/interface.py
@@ -348,7 +348,7 @@ def adjoint_derivative(dx=None, dy=None, dz=None,
             # but use slack variables too
             raise NotImplementedError
         else:
-            raise RuntimeError(f"Unrecognized differentiation mode: {diff_mode}")
+            raise RuntimeError("Unrecognized differentiation mode: {}".format(diff_mode))
 
         # Extract derivatives
         rows, cols = P_idx