diff --git a/phpseclib/File/X509.php b/phpseclib/File/X509.php
index 3d4b1fcf..3a694f29 100644
--- a/phpseclib/File/X509.php
+++ b/phpseclib/File/X509.php
@@ -1650,7 +1650,10 @@ class File_X509
                    corresponding to the extension type identified by extnID */
                 $map = $this->_getMapping($id);
                 if (!is_bool($map)) {
-                    $mapped = $asn1->asn1map($decoded[0], $map, array('iPAddress' => array($this, '_decodeIP')));
+                    $decoder = $id == 'id-ce-nameConstraints' ?
+                        array($this, '_decodeNameConstraintIP') :
+                        array($this, '_decodeIP');
+                    $mapped = $asn1->asn1map($decoded[0], $map, array('iPAddress' => $decoder));
                     $value = $mapped === false ? $decoded[0] : $mapped;
 
                     if ($id == 'id-ce-certificatePolicies') {
@@ -2555,18 +2558,36 @@ class File_X509
         return long2ip($ip);
     }
 
+    /**
+     * Decodes an IP address in a name constraints extension
+     *
+     * Takes in a base64 encoded "blob" and returns a human readable IP address / mask
+     *
+     * @param string $ip
+     * @access private
+     * @return array
+     */
+    function _decodeNameConstraintIP($ip)
+    {
+        $ip = base64_decode($ip);
+        list(, $ip, $mask) = unpack('N2', $ip);
+        return [long2ip($ip), long2ip($mask)];
+    }
+
     /**
      * Encodes an IP address
      *
      * Takes a human readable IP address into a base64-encoded "blob"
      *
-     * @param string $ip
+     * @param string|array $ip
      * @access private
      * @return string
      */
     function _encodeIP($ip)
     {
-        return base64_encode(pack('N', ip2long($ip)));
+        return is_string($ip) ?
+            base64_encode(pack('N', ip2long($ip))) :
+            base64_encode(pack('NN', ip2long($ip[0]), ip2long($ip[1])));
     }
 
     /**
diff --git a/tests/Unit/File/X509/X509Test.php b/tests/Unit/File/X509/X509Test.php
index 60509a68..d8641834 100644
--- a/tests/Unit/File/X509/X509Test.php
+++ b/tests/Unit/File/X509/X509Test.php
@@ -691,4 +691,46 @@ A9bhRA0cVk7bAEU2c44CYg==
 
         $this->assertFalse($r);
     }
+
+    /**
+     * @group github1387
+     */
+    public function testNameConstraintIP()
+    {
+        $x509 = new File_X509();
+        $r = $x509->loadX509('-----BEGIN CERTIFICATE-----
+MIIGcDCCBVigAwIBAgIQRUgJC4ec7yFWcqzT3mwbWzANBgkqhkiG9w0BAQwFADB1MQswCQYDVQQG
+EwJFRTEiMCAGA1UECgwZQVMgU2VydGlmaXRzZWVyaW1pc2tlc2t1czEoMCYGA1UEAwwfRUUgQ2Vy
+dGlmaWNhdGlvbiBDZW50cmUgUm9vdCBDQTEYMBYGCSqGSIb3DQEJARYJcGtpQHNrLmVlMCAXDTE1
+MTIxNzEyMzg0M1oYDzIwMzAxMjE3MjM1OTU5WjBjMQswCQYDVQQGEwJFRTEiMCAGA1UECgwZQVMg
+U2VydGlmaXRzZWVyaW1pc2tlc2t1czEXMBUGA1UEYQwOTlRSRUUtMTA3NDcwMTMxFzAVBgNVBAMM
+DkVTVEVJRC1TSyAyMDE1MIICIjANBgkqhkiG9w0BAQEFAAOCAg8AMIICCgKCAgEA0oH61NDxbdW9
+k8nLA1qGaL4B7vydod2Ewp/STBZB3wEtIJCLdkpEsS8pXfFiRqwDVsgGGbu+Q99trlb5LI7yi7rI
+kRov5NftBdSNPSU5rAhYPQhvZZQgOwRaHa5Ey+BaLJHmLqYQS9hQvQsCYyws+xVvNFUpK0pGD64i
+ycqdMuBl/nWq3fLuZppwBh0VFltm4nhr/1S0R9TRJpqFUGbGr4OK/DwebQ5PjhdS40gCUNwmC7fP
+Q4vIH+x+TCk2aG+u3MoAz0IrpVWqiwzG/vxreuPPAkgXeFCeYf6fXLsGz4WivsZFbph2pMjELu6s
+ltlBXfAG3fGv43t91VXicyzR/eT5dsB+zFsW1sHV+1ONPr+qzgDxCH2cmuqoZNfIIq+buob3eA8e
+e+XpJKJQr+1qGrmhggjvAhc7m6cU4x/QfxwRYhIVNhJf+sKVThkQhbJ9XxuKk3c18wymwL1mpDD0
+PIGJqlssMeiuJ4IzagFbgESGNDUd4icm0hQT8CmQeUm1GbWeBYseqPhMQX97QFBLXJLVy2SCyoAz
+7Bq1qA43++EcibN+yBc1nQs2Zoq8ck9MK0bCxDMeUkQUz6VeQGp69ImOQrsw46qTz0mtdQrMSbnk
+XCuLan5dPm284J9HmaqiYi6j6KLcZ2NkUnDQFesBVlMEm+fHa2iR6lnAFYZ06UECAwEAAaOCAgow
+ggIGMB8GA1UdIwQYMBaAFBLyWj7qVhy/zQas8fElyalL1BSZMB0GA1UdDgQWBBSzq4i8mdVipIUq
+CM20HXI7g3JHUTAOBgNVHQ8BAf8EBAMCAQYwdwYDVR0gBHAwbjAIBgYEAI96AQIwCQYHBACL7EAB
+AjAwBgkrBgEEAc4fAQEwIzAhBggrBgEFBQcCARYVaHR0cHM6Ly93d3cuc2suZWUvQ1BTMAsGCSsG
+AQQBzh8BAjALBgkrBgEEAc4fAQMwCwYJKwYBBAHOHwEEMBIGA1UdEwEB/wQIMAYBAf8CAQAwQQYD
+VR0eBDowOKE2MASCAiIiMAqHCAAAAAAAAAAAMCKHIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
+AAAAAAAAMCcGA1UdJQQgMB4GCCsGAQUFBwMJBggrBgEFBQcDAgYIKwYBBQUHAwQwfAYIKwYBBQUH
+AQEEcDBuMCAGCCsGAQUFBzABhhRodHRwOi8vb2NzcC5zay5lZS9DQTBKBggrBgEFBQcwAoY+aHR0
+cDovL3d3dy5zay5lZS9jZXJ0cy9FRV9DZXJ0aWZpY2F0aW9uX0NlbnRyZV9Sb290X0NBLmRlci5j
+cnQwPQYDVR0fBDYwNDAyoDCgLoYsaHR0cDovL3d3dy5zay5lZS9yZXBvc2l0b3J5L2NybHMvZWVj
+Y3JjYS5jcmwwDQYJKoZIhvcNAQEMBQADggEBAHRWDGI3P00r2sOnlvLHKk9eE7X93eT+4e5TeaQs
+OpE5zQRUTtshxN8Bnx2ToQ9rgi18q+MwXm2f0mrGakYYG0bix7ZgDQvCMD/kuRYmwLGdfsTXwh8K
+uL6uSHF+U/ZTss6qG7mxCHG9YvebkN5Yj/rYRvZ9/uJ9rieByxw4wo7b19p22PXkAkXP5y3+qK/O
+et98lqwI97kJhiS2zxFYRk+dXbazmoVHnozYKmsZaSUvoYNNH19tpS7BLdsgi9KpbvQLb5ywIMq9
+ut3+b2Xvzq8yzmHMFtLIJ6Afu1jJpqD82BUAFcvi5vhnP8M7b974R18WCOpgNQvXDI+2/8ZINeU=
+-----END CERTIFICATE-----');
+        $r = $x509->saveX509($r);
+        $r = $x509->loadX509($r);
+        $this->assertSame($r['tbsCertificate']['extensions'][5]['extnValue']['excludedSubtrees'][1]['base']['iPAddress'], ['0.0.0.0', '0.0.0.0']);
+    }
 }