diff --git a/fslstats.cc b/fslstats.cc
index 8d6e5bf8f52453f0711624cf832b53ffbc35a43b..eb528b401b8bf34cedf239721624bf0afbb13c63 100644
--- a/fslstats.cc
+++ b/fslstats.cc
@@ -119,20 +119,27 @@ double nonzerostddev(const volume4D<float>& vol)
   return std::sqrt(var);
 }
 
-int generate_masks(volume4D<float> &mask, volume4D<float> &masknz, const volume4D<float> &vin,
-		   float& lthr, float& uthr) 
+int generate_masks(volume4D<float> &mask, volume4D<float> &masknz, const volume4D<float> &vin,float& lthr, float& uthr) 
 {
   if (!lthr_used) { lthr=vin.min()-1; }
   if (!uthr_used) { uthr=vin.max()+1; }
   mask = binarise(vin,lthr,uthr,exclusive);
-  if (mask.tsize()!=1) masknz = (binarise(vin,0.0f, 0.0f)-1.0f)*-1.0f*mask;
+  if (mask.tsize()!=1) {
+    masknz.reinitialize(mask.xsize(),mask.ysize(),mask.zsize(),mask.tsize());
+    for (int t=mask.mint(); t<=mask.maxt(); t++) 
+      masknz[t]=((binarise(vin[t],0.0f, 0.0f)-1.0f)*-1.0f*mask[t]);
+  }
   else masknz = (binarise(vin,0.0f, 0.0f)-1.0f)*-1.0f*mask[0];
   return 0;
 }
 
 int generate_masks(const volume4D<float> &mask, volume4D<float> &masknz, const volume4D<float> &vin)
 {
-  if (mask.tsize()!=1) masknz = (binarise(vin,0.0f, 0.0f)-1.0f)*-1.0f*mask;
+  if (mask.tsize()!=1) {
+    masknz.reinitialize(mask.xsize(),mask.ysize(),mask.zsize(),mask.tsize());
+    for (int t=mask.mint(); t<=mask.maxt(); t++) 
+      masknz[t]=((binarise(vin[t],0.0f, 0.0f)-1.0f)*-1.0f*mask[t]);
+  }
   else masknz = (binarise(vin,0.0f, 0.0f)-1.0f)*-1.0f*mask[0];
   return 0;
 }