diff --git a/dosma/scan_sequences/mri/mapss.py b/dosma/scan_sequences/mri/mapss.py
index df2c02b..d50f32f 100644
--- a/dosma/scan_sequences/mri/mapss.py
+++ b/dosma/scan_sequences/mri/mapss.py
@@ -223,13 +223,20 @@ def __fitting_helper(
         ys = [vol for _, vol in echo_info]
 
         # only calculate for focused region if a mask is available, this speeds up computation
-        mask = tissue.get_mask()
-        if mask_path is not None:
+        if tissue is not None:
+            mask = tissue.get_mask()
+        elif mask_path is not None:
             mask = (
                 fio_utils.generic_load(mask_path, expected_num_volumes=1)
                 if isinstance(mask_path, (str, os.PathLike))
                 else mask_path
             )
+        else:
+            mask = MedicalVolume(
+                volume=np.zeros_like(self.volumes[0].volume),
+                affine=volumes[0].affine,
+                headers=deepcopy(volumes[0].headers())
+            )
 
         mef = MonoExponentialFit(
             bounds=bounds,