diff --git a/mps/src/trace.c b/mps/src/trace.c
index 14da1f4b3e1..1b688a53293 100644
--- a/mps/src/trace.c
+++ b/mps/src/trace.c
@@ -1,12 +1,12 @@
 /* impl.c.trace: GENERIC TRACER IMPLEMENTATION
  *
- * $HopeName: MMsrc!trace.c(trunk.41) $
+ * $HopeName: MMsrc!trace.c(trunk.42) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  */
 
 #include "mpm.h"
 
-SRCID(trace, "$HopeName: MMsrc!trace.c(trunk.41) $");
+SRCID(trace, "$HopeName: MMsrc!trace.c(trunk.42) $");
 
 
 /* ScanStateCheck -- check consistency of a ScanState object */
@@ -587,8 +587,9 @@ static Res TraceScan(TraceSet ts, Rank rank,
                      Arena arena, Seg seg)
 {
   Res res;
-  ScanStateStruct ss;
   TraceId ti;
+  RefSet white;
+  ScanStateStruct ss;
 
   AVER(TraceSetCheck(ts));
   AVER(RankCheck(rank));
@@ -598,48 +599,53 @@ static Res TraceScan(TraceSet ts, Rank rank,
   AVER(TraceSetInter(ts, SegGrey(seg)) != TraceSetEMPTY);
   EVENT_UUPPP(TraceScan, ts, rank, arena, seg, &ss);
 
-  ss.rank = rank;
-  ss.traces = ts;
-  ss.fix = TraceFix;
-  ss.zoneShift = arena->zoneShift;
-  ss.unfixedSummary = RefSetEMPTY;
-  ss.fixedSummary = RefSetEMPTY;
-  ss.arena = arena;
-  ss.wasMarked = TRUE;
-  ss.white = RefSetEMPTY;
+  white = RefSetEMPTY;
   for(ti = 0; ti < TRACE_MAX; ++ti)
-    if(TraceSetIsMember(ss.traces, ti))
-      ss.white = RefSetUnion(ss.white, ArenaTrace(arena, ti)->white);
-  ss.sig = ScanStateSig;
-  AVERT(ScanState, &ss);
+    if(TraceSetIsMember(ts, ti))
+      white = RefSetUnion(white, ArenaTrace(arena, ti)->white);
 
-  /* Expose the segment to make sure we can scan it. */
-  ShieldExpose(arena, seg);
+  /* only scan a segment if it refers to the white set */
+  if (RefSetInter(white, SegSummary(seg)) != RefSetEMPTY) {
+    ss.rank = rank;
+    ss.traces = ts;
+    ss.fix = TraceFix;
+    ss.zoneShift = arena->zoneShift;
+    ss.unfixedSummary = RefSetEMPTY;
+    ss.fixedSummary = RefSetEMPTY;
+    ss.arena = arena;
+    ss.wasMarked = TRUE;
+    ss.white = white;
+    ss.sig = ScanStateSig;
+    AVERT(ScanState, &ss);
+    
+    /* Expose the segment to make sure we can scan it. */
+    ShieldExpose(arena, seg);
+    
+    res = PoolScan(&ss, SegPool(seg), seg);
 
-  res = PoolScan(&ss, SegPool(seg), seg);
-  if(res != ResOK)
-    goto failScan;
+    if(res != ResOK) {
+      ShieldCover(arena, seg);
+      return res;
+    }
+    
+    /* Cover the segment again, now it's been scanned. */
+    ShieldCover(arena, seg);
 
-  /* See design.mps.scan.summary.subset. */
-  AVER(RefSetSub(ss.unfixedSummary, SegSummary(seg)));
-
-  /* All objects on the segment have been scanned, so the scanned */
-  /* summary should replace the segment summary. */
-  SegSetSummary(seg, ScanStateSummary(&ss));
-
-  ss.sig = SigInvalid;                  /* just in case */
+    /* See design.mps.scan.summary.subset. */
+    AVER(RefSetSub(ss.unfixedSummary, SegSummary(seg)));
+    
+    /* All objects on the segment have been scanned, so the scanned */
+    /* summary should replace the segment summary. */
+    SegSetSummary(seg, ScanStateSummary(&ss));
+    
+    ss.sig = SigInvalid;                  /* just in case */
+  }
 
   /* The segment has been scanned, so remove the greyness from it. */
   SegSetGrey(seg, TraceSetDiff(SegGrey(seg), ts));
 
-  /* Cover the segment again, now it's been scanned. */
-  ShieldCover(arena, seg);
-
   return ResOK;
 
-failScan:
-  ShieldCover(arena, seg);
-  return res;
 }