(string->yaml "a:") throws a non-scanner-error

[baoti]

> (string->yaml "null:")
Second param is neither char-set, char, or predicate procedure. #<procedure:string-index> #<eof>
  context...:
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/scanner.rkt:1101:21: loop
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/scanner.rkt:1095:9: loop
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/scanner.rkt:1083:2: scan-plain
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/scanner.rkt:167:2: fetch-more-tokens
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/scanner.rkt:133:4: loop
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/scanner.rkt:131:2: check-token?
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/parser.rkt:89:2: parse-implicit-document-start
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/parser.rkt:53:2: check-event?
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/composer.rkt:56:2: get-single-node
   /home/sean/.racket/planet/300/5.3.1/cache/esilkensen/yaml.plt/2/1/private/constructor.rkt:60:2: get-single-data
   /usr/share/racket/collects/racket/private/misc.rkt:87:7
> 

[esilkensen]

Fix pushed to github.

> (string->yaml "null:")
'#hash((null . null))