diff --git a/phpseclib/File/ASN1.php b/phpseclib/File/ASN1.php
index da70d415..2774e7e8 100644
--- a/phpseclib/File/ASN1.php
+++ b/phpseclib/File/ASN1.php
@@ -950,6 +950,9 @@ class File_ASN1
             case FILE_ASN1_TYPE_INTEGER:
             case FILE_ASN1_TYPE_ENUMERATED:
                 if (!isset($mapping['mapping'])) {
+                    if (is_numeric($source)) {
+                        $source = new Math_BigInteger($source);
+                    }
                     $value = $source->toBytes(true);
                 } else {
                     $value = array_search($source, $mapping['mapping']);