diff --git a/index.html b/index.html
index 83853c5a6..4ac67131b 100644
--- a/index.html
+++ b/index.html
@@ -2248,8 +2248,11 @@ <h3>Proofs (e.g., Signatures)</h3>
           or key purpose.</li>
           <li>The key MUST NOT be revoked or expired.</li>
           <li>The cryptographic signature MUST be valid.</li>
-          <li>If a <code>proofPurpose</code> exists, it MUST be a valid value
-          per the cryptographic suite.</li>
+          <li>
+            If the cryptographic suite expects a <code>proofPurpose</code>,
+            it MUST exist and be a valid value (such as
+            <code>credentialIssuance</code>) per the cryptographic suite.
+          </li>
         </ul>
 
         <p class="note">