diff --git a/config/config.php b/config/config.php
index 90d321c..397b2e3 100644
--- a/config/config.php
+++ b/config/config.php
@@ -12,6 +12,11 @@
  * @author Martin Auswöger <martin@madeyourday.co>
  */
 
+// Backwards compatibility for Contao < 3.5.1
+if (!class_exists('StringUtil') && class_exists('String')) {
+	class_alias('String', 'StringUtil');
+}
+
 $GLOBALS['TL_HOOKS']['executePostActions'][] = array('MadeYourDay\\Contao\\ThemeAssistant', 'executePostActionsHook');
 
 $GLOBALS['BE_MOD']['design']['rocksolid_theme_assistant'] = array(
diff --git a/src/MadeYourDay/Contao/ThemeAssistant.php b/src/MadeYourDay/Contao/ThemeAssistant.php
index cca5cfa..b53d17e 100644
--- a/src/MadeYourDay/Contao/ThemeAssistant.php
+++ b/src/MadeYourDay/Contao/ThemeAssistant.php
@@ -546,7 +546,7 @@ public function onsubmitCallback(\DataContainer $dc)
 								$file = \FilesModel::findByPk($value);
 							}
 							else {
-								$file = \FilesModel::findByUuid(\String::uuidToBin($value));
+								$file = \FilesModel::findByUuid(\StringUtil::uuidToBin($value));
 							}
 						}
 						if ($file) {
@@ -563,7 +563,7 @@ public function onsubmitCallback(\DataContainer $dc)
 								$file = \FilesModel::findByPk($value);
 							}
 							else {
-								$file = \FilesModel::findByUuid(\String::uuidToBin($value));
+								$file = \FilesModel::findByUuid(\StringUtil::uuidToBin($value));
 							}
 						}
 						if ($file) {
diff --git a/src/MadeYourDay/Contao/ThemeAssistantDataContainer.php b/src/MadeYourDay/Contao/ThemeAssistantDataContainer.php
index 91aba3f..88a652c 100644
--- a/src/MadeYourDay/Contao/ThemeAssistantDataContainer.php
+++ b/src/MadeYourDay/Contao/ThemeAssistantDataContainer.php
@@ -46,7 +46,7 @@ public function __construct($strTable, $arrModule = array())
 			foreach ($GLOBALS['TL_DCA'][$this->strTable]['config']['onload_callback'] as $callback) {
 				if (is_array($callback)) {
 					$this->import($callback[0]);
-					$this->$callback[0]->$callback[1]($this);
+					$this->{$callback[0]}->{$callback[1]}($this);
 				}
 			}
 		}
@@ -315,7 +315,7 @@ public function edit()
 						foreach ($GLOBALS['TL_DCA'][$this->strTable]['fields'][$this->strField]['load_callback'] as $callback) {
 							if (is_array($callback)) {
 								$this->import($callback[0]);
-								$this->varValue = $this->$callback[0]->$callback[1]($this->varValue, $this);
+								$this->varValue = $this->{$callback[0]}->{$callback[1]}($this->varValue, $this);
 							}
 						}
 					}
@@ -372,7 +372,7 @@ public function edit()
 			if (is_array($GLOBALS['TL_DCA'][$this->strTable]['config']['onsubmit_callback'])) {
 				foreach ($GLOBALS['TL_DCA'][$this->strTable]['config']['onsubmit_callback'] as $callback) {
 					$this->import($callback[0]);
-					$this->$callback[0]->$callback[1]($this);
+					$this->{$callback[0]}->{$callback[1]}($this);
 				}
 			}