diff --git a/src/classes/PMsrHandler.cpp b/src/classes/PMsrHandler.cpp
index c89426a7..3d117f33 100644
--- a/src/classes/PMsrHandler.cpp
+++ b/src/classes/PMsrHandler.cpp
@@ -7039,10 +7039,12 @@ void PMsrHandler::MakeDetectorGroupingString(TString str, PIntVector &group, TSt
   UInt_t i=0, j=0;
   do {
     j = i;
-    while (group[j]+1 == group[j+1]) {
-      j++;
-      if (j == group.size()-1)
-        break;
+    if (j+1 < group.size()) {
+      while (group[j]+1 == group[j+1]) {
+        j++;
+        if (j == group.size()-1)
+          break;
+      }
     }
 
     if (j >= i+2) {