Index: /trunk/ppStack/src/ppStackMatch.c
===================================================================
--- /trunk/ppStack/src/ppStackMatch.c	(revision 20712)
+++ /trunk/ppStack/src/ppStackMatch.c	(revision 20713)
@@ -382,6 +382,6 @@
         psLogMsg("ppStack", PS_LOG_INFO, "Correcting convolved image background by %lf (+/- %lf)",
                  psStatsGetValue(bg, PS_STAT_ROBUST_MEDIAN), psStatsGetValue(bg, PS_STAT_ROBUST_STDEV));
-        (void)psBinaryOp(readout->image, readout->image, "+",
-                         psScalarAlloc(- psStatsGetValue(bg, PS_STAT_ROBUST_MEDIAN), PS_TYPE_F32));
+        (void)psBinaryOp(readout->image, readout->image, "-",
+                         psScalarAlloc(psStatsGetValue(bg, PS_STAT_ROBUST_MEDIAN), PS_TYPE_F32));
         *weighting = 1.0 / PS_SQR(psStatsGetValue(bg, PS_STAT_ROBUST_STDEV));
         psMetadataAddF32(readout->analysis, PS_LIST_TAIL, "PPSTACK.WEIGHTING", 0,