From 8271192f6329777cbcd59889f9f542352acab540 Mon Sep 17 00:00:00 2001
From: Andreas Suter <andreas.suter@psi.ch>
Date: Mon, 12 Apr 2021 07:50:37 +0200
Subject: [PATCH] make sure that group has indeed any elements.

---
 src/classes/PMsrHandler.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/classes/PMsrHandler.cpp b/src/classes/PMsrHandler.cpp
index 7794de21..84408668 100644
--- a/src/classes/PMsrHandler.cpp
+++ b/src/classes/PMsrHandler.cpp
@@ -6668,6 +6668,9 @@ void PMsrHandler::MakeDetectorGroupingString(TString str, PIntVector &group, TSt
     str = "";
   }
 
+  if (group.size()==0)
+    return;
+
   UInt_t i=0, j=0;
   do {
     j = i;