diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 76bd58d306defa114b5df239cb8a24e4bcd14be8..2a39c244ee60ee4b8922c078f200f4c2e65d72d4 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -14991,7 +14991,7 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        <entry><type>bigint</type></entry>
        <entry>
         Disk space used by the specified fork (<literal>'main'</literal>,
-        <literal>'fsm'</literal> or <literal>'vm'</>)
+        <literal>'fsm'</literal>, <literal>'vm'</>, or <literal>'init'</>)
         of the specified table or index
        </entry>
       </row>
@@ -15125,6 +15125,12 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
        (see <xref linkend="storage-vm">) associated with the relation.
       </para>
      </listitem>
+     <listitem>
+      <para>
+       <literal>'init'</literal> returns the size of the initialization fork,
+       if any, (see <xref linkend="storage-init">) associated with the relation.
+      </para>
+     </listitem>
     </itemizedlist>
    </para>
 
diff --git a/src/backend/catalog/catalog.c b/src/backend/catalog/catalog.c
index bd56534be5b29301299d60b80dc6eb1a6745c9db..03671104d0942c69b8f5c9f2295fd341beb0ef63 100644
--- a/src/backend/catalog/catalog.c
+++ b/src/backend/catalog/catalog.c
@@ -75,7 +75,8 @@ forkname_to_number(char *forkName)
 	ereport(ERROR,
 			(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 			 errmsg("invalid fork name"),
-			 errhint("Valid fork names are \"main\", \"fsm\", and \"vm\".")));
+			 errhint("Valid fork names are \"main\", \"fsm\", "
+					 "\"vm\", and \"init\".")));
 	return InvalidForkNumber;	/* keep compiler quiet */
 }