Specifically mention value must be a string.
--- a/spec/latest/json-ld-api/index.html Thu Feb 14 11:18:21 2013 -0500
+++ b/spec/latest/json-ld-api/index.html Thu Feb 14 12:09:21 2013 -0500
@@ -1940,9 +1940,9 @@
</ol>
</li>
<li>
- If <em>result</em> is <tref>null</tref> and <em>value</em> contains a
- colon (<code>:</code>), then it is either an <tref>absolute IRI</tref> or
- a <tref>compact IRI</tref>:
+ If <em>result</em> is <tref>null</tref> and <em>value</em> is a
+ <tref>string</tref> that contains a colon (<code>:</code>), then it is
+ either an <tref>absolute IRI</tref> or a <tref>compact IRI</tref>:
<ol class="algorithm">
<li>
Split <em>value</em> into a <tref>prefix</tref> and <em>suffix</em>