diff --git a/src/roi/thresholdToMask.m b/src/roi/thresholdToMask.m
index 7796ef7..7e896e9 100644
--- a/src/roi/thresholdToMask.m
+++ b/src/roi/thresholdToMask.m
@@ -48,7 +48,7 @@
 
   % add peakThreshold and clusterSizeInfo to desc
   if ~isfield(bf.entities, 'desc')
-    bf.entities.desc = '';
+    bf.entities(1).desc = '';
   end
   descSuffix = sprintf('p%05.2f', peakThreshold);
   if clusterSize > 0
@@ -58,7 +58,7 @@
   bf.entities.desc = [bf.entities.desc descSuffix];
 
   if isempty(bf.extension)
-    bf.extension = ".nii";
+    bf.extension = '.nii';
   end
 
   bf = bf.update();