Index: trunk/ippTools/src/pxtools.c
===================================================================
--- trunk/ippTools/src/pxtools.c	(revision 28043)
+++ trunk/ippTools/src/pxtools.c	(revision 28055)
@@ -125,9 +125,11 @@
     *software_ver = pxMergeCodeVersions(*software_ver,this_version);
 
-    *maskfrac_static = ((*maskfrac_static * *maskfrac_npix) + (this_npix * this_static)) / (this_npix + *maskfrac_npix);
-    *maskfrac_dynamic = ((*maskfrac_dynamic * *maskfrac_npix) + (this_npix * this_dynamic)) / (this_npix + *maskfrac_npix);
-    *maskfrac_magic = ((*maskfrac_magic * *maskfrac_npix) + (this_npix * this_magic)) / (this_npix + *maskfrac_npix);
-    *maskfrac_advisory = ((*maskfrac_advisory * *maskfrac_npix) + (this_npix * this_advisory)) / (this_npix + *maskfrac_npix);
-    *maskfrac_npix += this_npix;
+    if (this_npix > 0) {
+      *maskfrac_static = ((*maskfrac_static * *maskfrac_npix) + (this_npix * this_static)) / (this_npix + *maskfrac_npix);
+      *maskfrac_dynamic = ((*maskfrac_dynamic * *maskfrac_npix) + (this_npix * this_dynamic)) / (this_npix + *maskfrac_npix);
+      *maskfrac_magic = ((*maskfrac_magic * *maskfrac_npix) + (this_npix * this_magic)) / (this_npix + *maskfrac_npix);
+      *maskfrac_advisory = ((*maskfrac_advisory * *maskfrac_npix) + (this_npix * this_advisory)) / (this_npix + *maskfrac_npix);
+      *maskfrac_npix += this_npix;
+    }
   }
   psFree(output);