(read-json "")
</code></pre>
which throws the exception
<pre><code>java.lang.IllegalArgumentException: Value out of range for char: -1
</code></pre>
This is caused by the .read method of the Reader returning -1 when the end is reached. (.read returns an int.) This value is then passed to 'char', which tries to make the corresponding character from the code point number. However, -1 is not a valid code point, so it throws an exception.

A solution is to check for equality to -1 before trying to convert to a character. I also have an idea for a helper function that could be used:
<pre><code>(defn- read-char
  "Reads a character from a Reader. Returns nil if the end of the stream is reached"
  [stream]
  (let [x (.read stream)]
    (if (not= x -1)
      (char x))))