diff --git a/question/type/shortanswer/question.php b/question/type/shortanswer/question.php
index ba1dfc786dc..07b1ab00fb6 100644
--- a/question/type/shortanswer/question.php
+++ b/question/type/shortanswer/question.php
@@ -117,7 +117,7 @@ class qtype_shortanswer_question extends question_graded_by_strategy
      * @return string the normalised string.
      */
     protected static function safe_normalize($string) {
-        if (!$string) {
+        if ($string === '') {
             return '';
         }
 
@@ -126,7 +126,7 @@ class qtype_shortanswer_question extends question_graded_by_strategy
         }
 
         $normalised = normalizer_normalize($string, Normalizer::FORM_C);
-        if (!$normalised) {
+        if (is_null($normalised)) {
             // An error occurred in normalizer_normalize, but we have no idea what.
             debugging('Failed to normalise string: ' . $string, DEBUG_DEVELOPER);
             return $string; // Return the original string, since it is the best we have.
diff --git a/question/type/shortanswer/tests/question_test.php b/question/type/shortanswer/tests/question_test.php
index d287ff41ff5..5dc15ee694a 100644
--- a/question/type/shortanswer/tests/question_test.php
+++ b/question/type/shortanswer/tests/question_test.php
@@ -120,6 +120,18 @@ class qtype_shortanswer_question_test extends advanced_testcase {
         }
     }
 
+    public function test_compare_0_with_wildcard() {
+        // Test the classic PHP problem case with '0'.
+        $this->assertTrue((bool)qtype_shortanswer_question::compare_string_with_wildcard(
+                '0', '0', false));
+        $this->assertTrue((bool)qtype_shortanswer_question::compare_string_with_wildcard(
+                '0', '0*', false));
+        $this->assertTrue((bool)qtype_shortanswer_question::compare_string_with_wildcard(
+                '0', '*0', false));
+        $this->assertTrue((bool)qtype_shortanswer_question::compare_string_with_wildcard(
+                '0', '*0*', false));
+    }
+
     public function test_is_complete_response() {
         $question = test_question_maker::make_question('shortanswer');