aggregation expressions don't accept BSON decimal as numeric type

db.foo.aggregate({$addFields:{x:{$substr:[{$literal:"xxxxxxxx"}, 0, NumberDecimal("5")]}}})

2018-09-10T14:40:15.878-0400 E QUERY    [js] Error: command failed: {

	"ok" : 0,

	"errmsg" : "$substrBytes:  length must be a numeric type (is BSON type decimal)",

	"code" : 16035,

	"codeName" : "Location16035"

} : aggregate failed 

// same for other string functions

[js] Error: command failed: {

    "ok" : 0,

    "errmsg" : "$substrBytes:  length must be a numeric type (is BSON type decimal)",

    "code" : 16035,

    "codeName" : "Location16035"

} : aggregate failed :