Index: /trunk/ppMerge/src/ppMergeCombine.c
===================================================================
--- /trunk/ppMerge/src/ppMergeCombine.c	(revision 7743)
+++ /trunk/ppMerge/src/ppMergeCombine.c	(revision 7744)
@@ -85,5 +85,8 @@
                     // Only reading and writing the first readout in each cell (plane 0)
                     if (pmReadoutReadNext(stack->data[k], fits, 0, options->rows)) {
-                        pmReadoutSetMask(stack->data[k]);
+                        // For anything except BIAS and DARK, we want a mask
+                        if (options->scale || options->zero) {
+                            pmReadoutSetMask(stack->data[k]);
+                        }
                         numRead++;
                     }