diff --git a/reference/exec/functions/proc-open.xml b/reference/exec/functions/proc-open.xml
index 70d9fccfc8..54e0f2dc6b 100644
--- a/reference/exec/functions/proc-open.xml
+++ b/reference/exec/functions/proc-open.xml
@@ -15,9 +15,9 @@
       <methodparam><type>string</type><parameter>cmd</parameter></methodparam>
       <methodparam><type>array</type><parameter>descriptorspec</parameter></methodparam>
       <methodparam><type>array</type><parameter role="reference">pipes</parameter></methodparam>
-      <methodparam choice="opt"><type>string</type><parameter>cwd</parameter></methodparam>
-      <methodparam choice="opt"><type>array</type><parameter>env</parameter></methodparam>
-      <methodparam choice="opt"><type>array</type><parameter>other_options</parameter></methodparam>
+      <methodparam choice="opt"><type>string</type><parameter>cwd</parameter><initializer>&null;</initializer></methodparam>
+      <methodparam choice="opt"><type>array</type><parameter>env</parameter><initializer>&null;</initializer></methodparam>
+      <methodparam choice="opt"><type>array</type><parameter>other_options</parameter><initializer>&null;</initializer></methodparam>
      </methodsynopsis>
     <para>
      <function>proc_open</function> is similar to <function>popen</function>