diff --git a/reference/image/functions/imagecolortransparent.xml b/reference/image/functions/imagecolortransparent.xml
index 3c94004684..881f38e216 100644
--- a/reference/image/functions/imagecolortransparent.xml
+++ b/reference/image/functions/imagecolortransparent.xml
@@ -37,7 +37,9 @@
   &reftitle.returnvalues;
   <para>
    The identifier of the new (or current, if none is specified)
-   transparent color is returned.
+   transparent color is returned. If <parameter>color</parameter>
+   is not specified, and the image has no transparent color, the
+   returned identifier will be -1.
   </para>
  </refsect1>
  <refsect1 role="examples">