diff --git a/reference/image/functions/getimagesize.xml b/reference/image/functions/getimagesize.xml
index df310aee08..8127ff22cc 100644
--- a/reference/image/functions/getimagesize.xml
+++ b/reference/image/functions/getimagesize.xml
@@ -273,6 +273,18 @@ if (isset($info["APP13"])) {
  <refsect1 role="notes">
   &reftitle.notes;
   &note.gd.notrequired;
+  <note>
+   <para>
+    This function expects <parameter>filename</parameter> to be a valid
+    image file. If a non-image file is supplied, it may be incorrectly
+    detected as an image and the function will return successfully.
+   </para>
+   <para>
+    Do not use <function>getimagesize</function> to check that a given
+    file is a valid image. Use a purpose-built solution such as the
+    <link linkend="book.fileinfo">Fileinfo</link> extension instead.
+   </para>
+  </note>
  </refsect1>
  
  <refsect1 role="seealso">