diff --git a/fslmeants.cc b/fslmeants.cc
index fd3d211f3ba3132926e3a267dd62746ffdc6ce50..87921294a8d744f3ba3c9d84bf500452ea1aee50 100644
--- a/fslmeants.cc
+++ b/fslmeants.cc
@@ -42,11 +42,12 @@ int main(int argc,char *argv[])
   volume4D<float> vin;
   read_volume4D(vin,argv[1]);
 
-  bool use_mask = false;
   volume<float> mask;
   if (argc>=4) {
-    use_mask = true;
     read_volume(mask,argv[3]);
+  } else {
+    mask = vin[0];
+    mask = 1.0;
   }
 
   if (!samesize(vin[0],mask)) {
@@ -63,8 +64,7 @@ int main(int argc,char *argv[])
   for (int z=mask.minz(); z<mask.maxz(); z++) {
     for (int y=mask.miny(); y<mask.maxy(); y++) {
       for (int x=mask.minx(); x<mask.maxx(); x++) {
-	if ( (!use_mask) || (fabs(mask(x,y,z))>1e-8)) {  
-	    // arbitrary "0" threshold
+	if (fabs(mask(x,y,z))>1e-8) {  // arbitrary "0" threshold
 	  meants += vin.voxelts(x,y,z);
 	  num++;
 	}
@@ -72,6 +72,8 @@ int main(int argc,char *argv[])
     }
   }
 
+  cout << "Number of voxels used = " << num << endl;
+
   // normalise for number of valid entries
   meants /= (float) num;