diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4bb5ae3fd7e..bd03ea0e102 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+- Fix PHP8 fatal error when parsing a malformed BODYSTRUCTURE (#9171)
+
+## Release 1.6.4
+
 - Fix PHP8 warnings (#9142, #9160)
 - Fix default 'mime.types' path on Windows (#9113)
 - Managesieve: Fix javascript error when relational or spamtest extension is not enabled (#9139)
diff --git a/program/lib/Roundcube/rcube_imap.php b/program/lib/Roundcube/rcube_imap.php
index d5ec7d73546..6689bbf8348 100644
--- a/program/lib/Roundcube/rcube_imap.php
+++ b/program/lib/Roundcube/rcube_imap.php
@@ -2164,7 +2164,7 @@ protected function structure_part($part, $count = 0, $parent = '', $mime_headers
         }
 
         // read content encoding
-        if (!empty($part[5])) {
+        if (!empty($part[5]) && !is_array($part[5])) {
             $struct->encoding = strtolower($part[5]);
             $struct->headers['content-transfer-encoding'] = $struct->encoding;
         }