Problem with FileWriteStruct

I'm trying to write a script which writes my trading history to a binary file. I tried to follow the example at https://docs.mql4.com/files/filewritestruct, but I keep getting the compile error "'tradeBuff' - parameter conversion not allowed OSR Trade History Recorder Binary.mq4 72 33"

 I can't figure out what the compiler is unhappy. Do you know why this won't compile?

#property strict

struct trade 
{
   string symbol;
   int direction;
   double lots;
   datetime openTime;
   datetime closeTime;
   double sl;
   double tp;
};


void OnStart()
{  
   trade tradeBuff[];
   string symbols[];
   int directions[];
   double lots[];
   datetime openTimes[];
   datetime closeTimes[];
   
   int size = OrdersHistoryTotal();
   ArrayResize(tradeBuff,size);
   ArrayResize(symbols,size);
   ArrayResize(directions,size);
   ArrayResize(lots,size);
   ArrayResize(openTimes,size);
   ArrayResize(closeTimes,size);
   
   for(int i = size - 1; i >= 0; i--)
   {
      if( OrderSelect(i, SELECT_BY_POS, MODE_HISTORY) )
      {
         if(OrderType()==6)
            continue;
         
         symbols[i] = OrderSymbol();
         directions[i] = OrderType();        
         lots[i] = OrderLots();
         openTimes[i] = OrderOpenTime();
         closeTimes[i] = OrderCloseTime();
      }
   }
   
   for(int i = size - 1; i >= 0; i--)
   {
      tradeBuff[i].symbol = symbols[i];
      tradeBuff[i].direction = directions[i];
      tradeBuff[i].lots = lots[i];
      tradeBuff[i].openTime = openTimes[i];
      tradeBuff[i].closeTime = closeTimes[i];
   }
 
   string file = "tradeHistory.bin";
   
   int handle = FileOpen(file,FILE_READ|FILE_WRITE|FILE_BIN|FILE_COMMON);
   
   if( handle != INVALID_HANDLE )
   {
      for(int i = size - 1; i >= 0; i--)
      {
         FileWriteStruct(handle,tradeBuff[i]);  // THIS IS WHERE THE COMPILE ERROR OCCURS
	// 'tradeBuff' - parameter conversion not allowed	OSR Trade History Recorder Binary.mq4	72	33
      }
   }   
   FileClose(handle);  
}