Index: trunk/Mars/mimage/MHillasExt.cc
===================================================================
--- trunk/Mars/mimage/MHillasExt.cc	(revision 19202)
+++ trunk/Mars/mimage/MHillasExt.cc	(revision 19203)
@@ -299,9 +299,14 @@
     const Double_t meandtw  = sumdtw/sumw;
 
-    fTimeSpread          = TMath::Sqrt(sumt2 /cnt  - meant *meant);
-    fTimeSpreadWeighted  = TMath::Sqrt(sumt2w/sumw - meantw*meantw);
-
-    fSlopeSpread         = TMath::Sqrt(sumdt2 /cnt  - meandt *meandt);
-    fSlopeSpreadWeighted = TMath::Sqrt(sumdt2w/sumw - meandtw*meandtw);
+    const Double_t rad_ts   = sumt2  /cnt  - meant *meant;
+    const Double_t rad_tsw  = sumt2w /sumw - meantw*meantw;
+    const Double_t rad_ss   = sumdt2 /cnt  - meandt *meandt;
+    const Double_t rad_ssw  = sumdt2w/sumw - meandtw*meandtw;
+
+    fTimeSpread          = rad_ts <0 ? 0 : TMath::Sqrt(rad_ts);
+    fTimeSpreadWeighted  = rad_tsw<0 ? 0 : TMath::Sqrt(rad_tsw);
+
+    fSlopeSpread         = rad_ss <0 ? 0 : TMath::Sqrt(rad_ss);
+    fSlopeSpreadWeighted = rad_ssw<0 ? 0 : TMath::Sqrt(rad_ssw);
 
     //