diff --git a/exercises/lt_exercise.php b/exercises/lt_exercise.php
index c658134fa94286f654705739ac223679ead486c2..a60b69a3bbe2fe95ebdad640d46e16abd321a916 100644
--- a/exercises/lt_exercise.php
+++ b/exercises/lt_exercise.php
@@ -81,36 +81,38 @@ class lt_exercise extends Exercise
 
             if ($musterLoesung === $studentSolution) {
                 $similarity_temp = 1;
-            } else if ($this->task['compare'] === 'levenshtein') {  // Levenshtein-Distanz
-                $string1 = mb_substr($studentSolution, 0, 255);
-                $string2 = mb_substr($musterLoesung, 0, 255);
-                $divisor = max(mb_strlen($string1), mb_strlen($string2));
-
-                $levenshtein = vips_levenshtein($string1, $string2) / $divisor;
-                $similarity_temp = 1 - $levenshtein;
-            } else if ($this->task['compare'] === 'soundex') {  // Soundex-Aussprache
-                $levenshtein = levenshtein(soundex($musterLoesung), soundex($studentSolution));
-
-                if ($levenshtein == 0) {
-                    $similarity_temp = 0.8;
-                } else if ($levenshtein == 1) {
-                    $similarity_temp = 0.6;
-                } else if ($levenshtein == 2) {
-                    $similarity_temp = 0.4;
-                } else if ($levenshtein == 3) {
-                    $similarity_temp = 0.2;
-                } else {// $levenshtein == 4
-                    $similarity_temp = 0;
-                }
-            } else if ($this->task['compare'] === 'numeric') {
-                $correct = normalizeFloat($answer['text'], $correct_unit);
-                $student = normalizeFloat($response[0], $student_unit);
-
-                if ($correct_unit === $student_unit) {
-                    if (abs($correct - $student) <= abs($correct * $this->task['epsilon'])) {
-                        $similarity_temp = 1;
-                    } else {
-                        $safe = true;
+            } else if (isset($this->task['compare'])) {
+                if ($this->task['compare'] === 'levenshtein') {  // Levenshtein-Distanz
+                    $string1 = mb_substr($studentSolution, 0, 255);
+                    $string2 = mb_substr($musterLoesung, 0, 255);
+                    $divisor = max(mb_strlen($string1), mb_strlen($string2));
+
+                    $levenshtein = vips_levenshtein($string1, $string2) / $divisor;
+                    $similarity_temp = 1 - $levenshtein;
+                } else if ($this->task['compare'] === 'soundex') {  // Soundex-Aussprache
+                    $levenshtein = levenshtein(soundex($musterLoesung), soundex($studentSolution));
+
+                    if ($levenshtein == 0) {
+                        $similarity_temp = 0.8;
+                    } else if ($levenshtein == 1) {
+                        $similarity_temp = 0.6;
+                    } else if ($levenshtein == 2) {
+                        $similarity_temp = 0.4;
+                    } else if ($levenshtein == 3) {
+                        $similarity_temp = 0.2;
+                    } else {// $levenshtein == 4
+                        $similarity_temp = 0;
+                    }
+                } else if ($this->task['compare'] === 'numeric') {
+                    $correct = normalizeFloat($answer['text'], $correct_unit);
+                    $student = normalizeFloat($response[0], $student_unit);
+
+                    if ($correct_unit === $student_unit) {
+                        if (abs($correct - $student) <= abs($correct * $this->task['epsilon'])) {
+                            $similarity_temp = 1;
+                        } else {
+                            $safe = true;
+                        }
                     }
                 }
             }