Index: trunk/MagicSoft/Mars/mimage/MNewImagePar.cc
===================================================================
--- trunk/MagicSoft/Mars/mimage/MNewImagePar.cc	(revision 8677)
+++ trunk/MagicSoft/Mars/mimage/MNewImagePar.cc	(revision 8695)
@@ -271,4 +271,8 @@
     fConcCOG /= hillas.GetSize();                        // [ratio]
 
+    // This can for example happen in case of Muon Rings
+    if (fConcCOG<0)
+        fConcCOG=0;
+
     // Concentration of signal contained in ellipse
     fConcCore /= hillas.GetSize();