Index: trunk/ppSub/src/ppSubBackground.c
===================================================================
--- trunk/ppSub/src/ppSubBackground.c	(revision 26982)
+++ trunk/ppSub/src/ppSubBackground.c	(revision 29003)
@@ -67,15 +67,17 @@
     for (int y = 0; y < numRows; y++) {
         for (int x = 0; x < numCols; x++) {
+	    // special case 1: NAN the masked pixels
             if (mask && mask->data.PS_TYPE_IMAGE_MASK_DATA[y][x] & maskBad) {
-                image->data.F32[y][x] = 0.0;
-            } else {
-                float value = psImageUnbinPixel(x + 0.5, y + 0.5, modelImage, binning); // Background value
-                if (!isfinite(value)) {
-                    image->data.F32[y][x] = NAN;
-                    mask->data.PS_TYPE_IMAGE_MASK_DATA[y][x] |= maskBad;
-                } else {
-                    image->data.F32[y][x] -= value;
-                }
-            }
+                image->data.F32[y][x] = NAN;
+		continue;
+            } 
+	    // special case 1: NAN & mask pixels without a valid background model
+	    float value = psImageUnbinPixel(x + 0.5, y + 0.5, modelImage, binning); // Background value
+	    if (!isfinite(value)) {
+	      image->data.F32[y][x] = NAN;
+	      mask->data.PS_TYPE_IMAGE_MASK_DATA[y][x] |= maskBad;
+	      continue;
+	    } 
+	    image->data.F32[y][x] -= value;
         }
     }