The Range header field ABNF:

  Range = byte-ranges-specifier / other-ranges-specifier  other-ranges-specifier = other-range-unit "=" other-range-set  other-range-set = 1*VCHAR

This is another misguided attempt to restrict syntax using ABNF in presence of extension points.

For instance,

bytes=abc

matchesother-ranges-specifier.

The proper fix for this is to rewrite the ABNF so it's properly layered - a generic one, plus an overlay for the only range unit we define.