MT5 error 10014: Invalid volume in the request

#include <Trade/Trade.mqh>
CTrade trade;

void OpenBuy(const double riskLots)
  {
   trade.Buy(riskLots, _Symbol);             // 0.0137 lots, straight from a risk formula
  }

#include <Trade/Trade.mqh>
CTrade trade;

double NormalizeVolume(const string symbol, const double lots)
  {
   const double step = SymbolInfoDouble(symbol, SYMBOL_VOLUME_STEP);
   const double vmin = SymbolInfoDouble(symbol, SYMBOL_VOLUME_MIN);
   const double vmax = SymbolInfoDouble(symbol, SYMBOL_VOLUME_MAX);
   double v = MathFloor(lots / step + 1e-9) * step;   // round down to a whole step
   if(v < vmin)
      return(0.0);                            // too small to trade: skip it
   return(MathMin(v, vmax));
  }

void OpenBuy(const double riskLots)
  {
   const double lots = NormalizeVolume(_Symbol, riskLots);
   if(lots > 0.0)
      trade.Buy(lots, _Symbol);
  }