Escaping variable in string

lang/en_utf8/debug.php

@@ -20,7 +20,7 @@
 $string['mustbeoveride'] = 'Abstract $a method must be overriden.';
 $string['morethanonerecordinfetch'] = 'Found more than one record in fetch() !';
 $string['noadminrole'] = 'No admin role could be found';
-$string['noactivityname'] = 'Page object derived from page_generic_activity but did not define $this->activityname';
+$string['noactivityname'] = 'Page object derived from page_generic_activity but did not define \$this->activityname';
 $string['noblocks'] = 'No blocks installed!';
 $string['nocate'] = 'No categories!';
 $string['notables'] = 'No Tables!';