diff --git a/question/type/calculated/question.php b/question/type/calculated/question.php
index 2936c05ba68..5e6d2e10d23 100644
--- a/question/type/calculated/question.php
+++ b/question/type/calculated/question.php
@@ -335,7 +335,11 @@ class qtype_calculated_variable_substituter {
      * @return string formtted number.
      */
     public function format_float($x, $length = null, $format = null) {
-        if (!is_null($length) && !is_null($format)) {
+        if (is_nan($x)) {
+            $x = 'NAN';
+        } else if (is_infinite($x)) {
+            $x = ($x < 0) ? '-INF' : 'INF';
+        } else if (!is_null($length) && !is_null($format)) {
             if ($format == '1' ) { // Answer is to have $length decimals.
                 // Decimal places.
                 $x = sprintf('%.' . $length . 'F', $x);
diff --git a/question/type/calculated/questiontype.php b/question/type/calculated/questiontype.php
index 39f293d1590..ba00ffa7739 100644
--- a/question/type/calculated/questiontype.php
+++ b/question/type/calculated/questiontype.php
@@ -1857,6 +1857,9 @@ function qtype_calculated_calculate_answer($formula, $individualdata,
         // Something went wrong, so just return NaN.
         $calculated->answer = NAN;
         return $calculated;
+    } else if (is_nan($answer) || is_infinite($answer)) {
+        $calculated->answer = $answer;
+        return $calculated;
     }
     if ('1' == $answerformat) { // Answer is to have $answerlength decimals.
         // Decimal places.
diff --git a/question/type/calculated/tests/questiontype_test.php b/question/type/calculated/tests/questiontype_test.php
index 46fcb70c31a..7fad0510426 100644
--- a/question/type/calculated/tests/questiontype_test.php
+++ b/question/type/calculated/tests/questiontype_test.php
@@ -238,4 +238,19 @@ class qtype_calculated_test extends advanced_testcase {
                         </pre>
                         '));
     }
+
+    public function test_calculate_answer_nan_inf() {
+        $answer = qtype_calculated_calculate_answer('acos(1.1)', [], 0.1, 1, 2, 2);
+        $this->assertIsObject($answer);
+        $this->assertNan($answer->answer);
+
+        $answer = qtype_calculated_calculate_answer('log(0.0)', [], 0.1, 1, 2, 2);
+        $this->assertIsObject($answer);
+        $this->assertInfinite($answer->answer); // Actually -INF.
+
+        // Dividing by zero is hard to test, so get +INF another way.
+        $answer = qtype_calculated_calculate_answer('abs(log(0.0))', [], 0.1, 1, 2, 2);
+        $this->assertIsObject($answer);
+        $this->assertInfinite($answer->answer);
+    }
 }
diff --git a/question/type/calculated/tests/variablesubstituter_test.php b/question/type/calculated/tests/variablesubstituter_test.php
index aff4ca7aaeb..4c813dae78d 100644
--- a/question/type/calculated/tests/variablesubstituter_test.php
+++ b/question/type/calculated/tests/variablesubstituter_test.php
@@ -134,4 +134,12 @@ class qtype_calculated_variable_substituter_test extends advanced_testcase {
         $this->assertSame('0,12', $vs->format_float(0.12345, 2, 2));
         $this->assertSame('0,0012', $vs->format_float(0.0012345, 4, 1));
     }
+
+    public function test_format_float_nan_inf() {
+        $vs = new qtype_calculated_variable_substituter([ ], '.');
+
+        $this->assertSame('NAN', $vs->format_float(NAN));
+        $this->assertSame('INF', $vs->format_float(INF));
+        $this->assertSame('-INF', $vs->format_float(-INF));
+    }
 }