diff --git a/dap/worker.py b/dap/worker.py
index b3bf7f7..591b206 100644
--- a/dap/worker.py
+++ b/dap/worker.py
@@ -201,7 +201,8 @@ def work(backend_address, accumulator_host, accumulator_port, visualisation_host
                         forceSendVisualisation = True
                         data_summed = None
                         n_aggregated_images = 1
-                data[~pixel_mask_pf] = np.nan
+                if pixel_mask_pf is not None:
+                    data[~pixel_mask_pf] = np.nan
 
             else:
                 data = image