diff --git a/src/cdtools/models/bragg_2d_ptycho.py b/src/cdtools/models/bragg_2d_ptycho.py
index b3869a9c..507323d0 100644
--- a/src/cdtools/models/bragg_2d_ptycho.py
+++ b/src/cdtools/models/bragg_2d_ptycho.py
@@ -285,13 +285,14 @@ def from_dataset(
         #   > dataset.sample_info['orientation'] > transmission geometry
         if surface_normal is not None:
             surface_normal = np.asarray(surface_normal)
-        elif scattering_mode.strip().lower() in {'t', 'transmission'}:
-            surface_normal = np.array([0.,0.,1.])
-        elif scattering_mode.strip().lower() in {'r', 'reflection'}:
-            outgoing_dir = np.cross(det_basis[:,0], det_basis[:,1])
-            outgoing_dir /= np.linalg.norm(outgoing_dir)
-            surface_normal = outgoing_dir + np.array([0.,0.,1.])
-            surface_normal /= np.linalg.norm(outgoing_dir)
+        elif isinstance(scattering_mode, str):
+            if scattering_mode.strip().lower() in {'t', 'transmission'}:
+                surface_normal = np.array([0.,0.,1.])
+            elif scattering_mode.strip().lower() in {'r', 'reflection'}:
+                outgoing_dir = np.cross(det_basis[:,0], det_basis[:,1])
+                outgoing_dir /= np.linalg.norm(outgoing_dir)
+                surface_normal = outgoing_dir + np.array([0.,0.,1.])
+                surface_normal /= np.linalg.norm(outgoing_dir)
         elif scattering_mode is not None:
             raise ValueError(
                 'Scattering mode must be either "transmission" ("t"), "reflection" ("r"), or the default of None.'