diff --git a/Control/AthenaMonitoringKernel/src/HistogramFiller/OfflineHistogramProvider.h b/Control/AthenaMonitoringKernel/src/HistogramFiller/OfflineHistogramProvider.h
index fa68491237438bd4f296459f4fadb9e5e3d4848e..22322a685b7372be2a0541b4f1b9c2d121d3cf88 100644
--- a/Control/AthenaMonitoringKernel/src/HistogramFiller/OfflineHistogramProvider.h
+++ b/Control/AthenaMonitoringKernel/src/HistogramFiller/OfflineHistogramProvider.h
@@ -75,7 +75,7 @@ namespace Monitored {
       if ( period == HistogramDef::RunPeriod::Run ) {
         lbString = "";
       } else if ( period == HistogramDef::RunPeriod::LowStat ) {
-        const unsigned lbBase = lumiBlock-((lumiBlock-1)%20);
+        const unsigned lbBase = lumiBlock-(((int64_t)lumiBlock-1)%20);
         lbString = "/lowStat_LB"+std::to_string(lbBase)+"-"+std::to_string(lbBase+19);
       } else {
         lbString = "/lb_"+std::to_string(lumiBlock);