diff --git a/reference/uopz/book.xml b/reference/uopz/book.xml
index c652ade0b4..a276ff75cc 100644
--- a/reference/uopz/book.xml
+++ b/reference/uopz/book.xml
@@ -29,6 +29,12 @@
  All of the activities supported are compatible with opcache
    </para>
   </note>
+  <caution>
+   <simpara>
+    PECL uopz 6.1.1 is not compatible with Xdebug &gt;= 2.9.4.
+    Later uopz versions are not compatible with Xdebug &lt; 2.9.4.
+   </simpara>
+  </caution>
  </preface>
 
  &reference.uopz.setup;