Selecting one of 84 characters purely randomly gives you 6.3923174227787602763 bits of entropy.<p>Selecting one of 84 characters by using modulus on a 32 bit random number gives you 6.3923174227787602889 bits of entropy.<p>It is a bug, but it is absolutely a non-problem.  The difference you would get from using 83 or 85 characters is many orders of magnitude larger; this is not something that needs such an enormous level of precision.