diff --git a/.travis.sh b/.travis.sh
new file mode 100644
index 00000000..d020add6
--- /dev/null
+++ b/.travis.sh
@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+if [[ $TRAVIS_PHP_VERSION = 7.* ]]; then
+	phpdbg -qrr ./vendor/bin/phpunit --coverage-clover build/logs/clover.xml
+else
+	./vendor/bin/phpunit --coverage-clover build/logs/clover.xml
+fi
diff --git a/.travis.yml b/.travis.yml
index 7a51c61f..1aa8c5d6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,16 +1,19 @@
 language: php
 
 php:
+  - 5.5
+  - 5.6
   - 7.0
 
 before_script:
+  - chmod a+x ./.travis.sh
   - composer self-update
   - composer install --no-ansi
   - wget https://scrutinizer-ci.com/ocular.phar
   - mkdir -p build/logs
 
 script:
-  - phpdbg -qrr ./vendor/bin/phpunit --coverage-clover build/logs/clover.xml
+  - ./.travis.sh
   - composer cs
 
 after_script:
diff --git a/src/API/NTLMSoapClient.php b/src/API/NTLMSoapClient.php
index 038127f1..41ddf0d4 100644
--- a/src/API/NTLMSoapClient.php
+++ b/src/API/NTLMSoapClient.php
@@ -106,18 +106,25 @@ public function __call($name, $args)
             $args[0] = $args[0]->toXmlObject();
         }
 
-        $this->__default_headers = array (
+        $headers = array (
             $this->ewsHeaders['version'],
-            $this->ewsHeaders['impersonation']
+            $this->ewsHeaders['impersonation'],
         );
 
         if (!in_array($name, $this->callsWithoutTimezone)) {
-            $this->__default_headers[] = $this->ewsHeaders['timezone'];
+            $headers[] = $this->ewsHeaders['timezone'];
         }
 
-        $response = parent::__call($name, $args);
-        $this->__default_headers = [];
-        return $response;
+        $headers = array_filter($headers, function ($header) {
+            if (!($header instanceof SoapHeader)) {
+                return false;
+            }
+
+            return true;
+        });
+
+        $this->__setSoapHeaders($headers);
+        return parent::__call($name, $args);
     }
 
     /**