PVS-Studio warning V636 for OpenMS

typedef size_t Size;
typedef double DoubleReal;

void updateMeanEstimate(const DoubleReal & x_t,
                        DoubleReal & mean_t, Size t)
{
  DoubleReal tmp(mean_t);
  tmp = mean_t + (1 / (t + 1)) * (x_t - mean_t);
  mean_t = tmp;
}

This suspicious code was found in OpenMS project by PVS-Studio static code analyzer.
Warning message is:
V636 The '1 / (t + 1)' expression was implicitly casted from 'int' type to 'double' type. Consider utilizing an explicit type cast to avoid the loss of a fractional part. An example: double A = (double)(X) / Y;. masstracedetection.c 129

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