diff --git a/LuaLibs/CalcToolPath.lua b/LuaLibs/CalcToolPath.lua
index 963002e..a4435c4 100644
--- a/LuaLibs/CalcToolPath.lua
+++ b/LuaLibs/CalcToolPath.lua
@@ -205,7 +205,8 @@ function CalcToolPath.Exec( nPartId)
          local nPrevId = EgtGetFirstInGroup( nTpathGrpId)
          local nCurrId = EgtGetNext( nPrevId or GDB_ID.NULL)
          k = 0
-         local nPrevShellNbr = tonumber( EgtGetName( nPrevId):sub(6))
+         local nPrevShellNbr
+         if nPrevId then nPrevShellNbr = tonumber( EgtGetName( nPrevId):sub(6)) end
          while nCurrId do
 
             local nCurrShellNbr = tonumber( EgtGetName( nCurrId):sub(6))