diff --git a/lib/openpgp.php b/lib/openpgp.php
index ae3b4b9..84f4fd6 100644
--- a/lib/openpgp.php
+++ b/lib/openpgp.php
@@ -424,13 +424,13 @@ class OpenPGP_SignaturePacket extends OpenPGP_Packet {
     $body = chr(4).chr($this->signature_type).chr($this->key_algorithm).chr($this->hash_algorithm);
 
     $hashed_subpackets = '';
-    foreach($this->hashed_subpackets as $p) {
+    foreach((array)$this->hashed_subpackets as $p) {
       $hashed_subpackets .= $p->to_bytes();
     }
     $body .= pack('n', strlen($hashed_subpackets)).$hashed_subpackets;
 
     $unhashed_subpackets = '';
-    foreach($this->unhashed_subpackets as $p) {
+    foreach((array)$this->unhashed_subpackets as $p) {
       $unhashed_subpackets .= $p->to_bytes();
     }
     $body .= pack('n', strlen($unhashed_subpackets)).$unhashed_subpackets;