diff --git a/src/engine/platform/genesisext.cpp b/src/engine/platform/genesisext.cpp
index 2997b9ca..4343f8ae 100644
--- a/src/engine/platform/genesisext.cpp
+++ b/src/engine/platform/genesisext.cpp
@@ -197,7 +197,7 @@ void DivPlatformGenesisExt::muteChannel(int ch, bool mute) {
   }
   isOpMuted[ch-2]=mute;
   
-  int ordch=orderedOps[ch];
+  int ordch=orderedOps[ch-2];
   DivInstrument* ins=parent->getIns(opChan[ch].ins);
   unsigned short baseAddr=chanOffs[2]|opOffs[ordch];
   DivInstrumentFM::Operator op=ins->fm.op[ordch];
diff --git a/src/engine/platform/ym2610ext.cpp b/src/engine/platform/ym2610ext.cpp
index 5a5e310a..8a180202 100644
--- a/src/engine/platform/ym2610ext.cpp
+++ b/src/engine/platform/ym2610ext.cpp
@@ -248,7 +248,7 @@ void DivPlatformYM2610Ext::muteChannel(int ch, bool mute) {
   }
   isOpMuted[ch-1]=mute;
   
-  int ordch=orderedOps[ch];
+  int ordch=orderedOps[ch-1];
   DivInstrument* ins=parent->getIns(opChan[ch].ins);
   unsigned short baseAddr=chanOffs[1]|opOffs[ordch];
   DivInstrumentFM::Operator op=ins->fm.op[ordch];