Index: /trunk/psModules/src/detrend/pmBias.c
===================================================================
--- /trunk/psModules/src/detrend/pmBias.c	(revision 13753)
+++ /trunk/psModules/src/detrend/pmBias.c	(revision 13754)
@@ -298,12 +298,16 @@
                 psImage *overscan = NULL; // Overscan image from iterator
                 while ((overscan = psListGetAndIncrement(iter))) {
-                    int diff = image->row0 - overscan->row0; // Offset between the two regions
+		    // the overscan and image may not be aligned.  pixels->data represents the 
+		    // image row pixels.  
+                    int diff = overscan->row0 - image->row0; // Offset between the two regions
                     for (int i = PS_MAX(0,diff); i < PS_MIN(image->numRows, overscan->numRows + diff); i++) {
-                        // i is row on overscan
+			int j = i - diff;
+                        // i is row on image
+			// j is row on overscan
                         psVector *values = pixels->data[i];
                         int index = values->n; // Index in the vector
                         values = psVectorRealloc(values, values->n + overscan->numCols);
                         values->n += overscan->numCols;
-                        memcpy(&values->data.F32[index], overscan->data.F32[i],
+                        memcpy(&values->data.F32[index], overscan->data.F32[j],
                                overscan->numCols * PSELEMTYPE_SIZEOF(PS_TYPE_F32));
                         index += overscan->numCols;