diff --git a/zUtil_Cxx/include/gtCxxPlaceSubVolOps.h b/zUtil_Cxx/include/gtCxxPlaceSubVolOps.h
index aa502bf20ee5b5ace66a4fce24ce897fcb088b48..bb2c4dadfe98c4bc8e9ecf08f098cab980b1c8dd 100644
--- a/zUtil_Cxx/include/gtCxxPlaceSubVolOps.h
+++ b/zUtil_Cxx/include/gtCxxPlaceSubVolOps.h
@@ -80,7 +80,7 @@ namespace GT3D {
     {
       if (voxel_ip)
       {
-        voxel_op = (-1 + (! voxel_op) * voxel_ip);
+        voxel_op = (-1 + (! voxel_op) * (voxel_ip + 1));
       }
     }
   protected:
@@ -92,7 +92,7 @@ namespace GT3D {
   {
     if (voxel_ip)
     {
-      voxel_op = (-1.0f + (! voxel_op) * voxel_ip);
+      voxel_op = (-1.0f + (! voxel_op) * (voxel_ip + 1.0f));
     }
   }
 
@@ -102,7 +102,7 @@ namespace GT3D {
   {
     if (voxel_ip)
     {
-      voxel_op = (-1.0 + (! voxel_op) * voxel_ip);
+      voxel_op = (-1.0 + (! voxel_op) * (voxel_ip + 1.0f));
     }
   }