Inconsistent URI Template parser messages

[zdne]

Parsing

## /message{$a,b,c}

results into

URI template expression "$a,b,c" contains invalid characters. Allowed characters for expressions are A-Z a-z 0-9 _ and percent encoded characters. :68:20

While

## /message{@a,b,c}

results into

warning: (12)  Unidentified expression. :68:20

Is there a reason for this? Can't we unify this?