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>