diff --git a/lib/grade/grade_grade.php b/lib/grade/grade_grade.php
index a083ffc6188..7476ac4ce28 100644
--- a/lib/grade/grade_grade.php
+++ b/lib/grade/grade_grade.php
@@ -718,6 +718,14 @@ class grade_grade extends grade_object {
     protected static function flatten_dependencies_array(&$dependson, &$dependencydepth) {
         // Flatten the nested dependencies - this will handle recursion bombs because it removes duplicates.
         $somethingchanged = true;
+        // First of all, delete any incorrect (not array or individual null) dependency, they aren't welcome.
+        // TODO: Maybe we should report about this happening, it shouldn't if all dependencies are correct and consistent.
+        foreach ($dependson as $itemid => $depends) {
+            $depends = is_array($depends) ? $depends : []; // Only arrays are accepted.
+            $dependson[$itemid] = array_filter($depends, function($val) { // Only not-null values are accepted.
+                return !is_null($val);
+            });
+        }
         while ($somethingchanged) {
             $somethingchanged = false;
 
@@ -725,7 +733,7 @@ class grade_grade extends grade_object {
                 // Make a copy so we can tell if it changed.
                 $before = $dependson[$itemid];
                 foreach ($depends as $subitemid => $subdepends) {
-                    $dependson[$itemid] = array_unique(array_merge($depends, $dependson[$subdepends]));
+                    $dependson[$itemid] = array_unique(array_merge($depends, $dependson[$subdepends] ?? []));
                     sort($dependson[$itemid], SORT_NUMERIC);
                 }
                 if ($before != $dependson[$itemid]) {
diff --git a/lib/grade/tests/grade_grade_test.php b/lib/grade/tests/grade_grade_test.php
index c9723d9e38c..488ae73df44 100644
--- a/lib/grade/tests/grade_grade_test.php
+++ b/lib/grade/tests/grade_grade_test.php
@@ -196,6 +196,11 @@ class core_grade_grade_testcase extends grade_base_testcase {
         $this->assertTrue($grade->is_hidden());
     }
 
+    /**
+     * Test grade_grade::flatten_dependencies_array()
+     *
+     * @covers \grade_grade::flatten_dependencies_array()
+     */
     public function test_flatten_dependencies() {
         // First test a simple normal case.
         $a = array(1 => array(2, 3), 2 => array(), 3 => array(4), 4 => array());
@@ -231,6 +236,66 @@ class core_grade_grade_testcase extends grade_base_testcase {
 
         test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
         $this->assertSame($expecteda, $a);
+
+        // Missing first level dependency.
+        $a = array(1 => array(2, 3), 3 => array(4), 4 => array());
+        $b = array();
+        $expecteda = array(1 => array(2, 3, 4), 3 => array(4), 4 => array());
+        $expectedb = array(1 => 1);
+
+        test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
+        $this->assertSame($expecteda, $a);
+        $this->assertSame($expectedb, $b);
+
+        // Missing 2nd level dependency.
+        $a = array(1 => array(2, 3), 2 => array(), 3 => array(4));
+        $b = array();
+        $expecteda = array(1 => array(2, 3, 4), 2 => array(), 3 => array(4));
+        $expectedb = array(1 => 1);
+
+        test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
+        $this->assertSame($expecteda, $a);
+        $this->assertSame($expectedb, $b);
+
+        // Null first level dependency.
+        $a = array(1 => array(2, null), 2 => array(3), 3 => array(4), 4 => array());
+        $b = array();
+        $expecteda = array(1 => array(2, 3, 4), 2 => array(3, 4), 3 => array(4), 4 => array());
+        $expectedb = array(1 => 2, 2 => 1);
+
+        test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
+        $this->assertSame($expecteda, $a);
+        $this->assertSame($expectedb, $b);
+
+        // Null 2nd level dependency.
+        $a = array(1 => array(2, 3), 2 => array(), 3 => array(4), 4 => array(null));
+        $b = array();
+        $expecteda = array(1 => array(2, 3, 4), 2 => array(), 3 => array(4), 4 => array());
+        $expectedb = array(1 => 1);
+
+        test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
+        $this->assertSame($expecteda, $a);
+        $this->assertSame($expectedb, $b);
+
+        // Straight null dependency.
+        $a = array(1 => array(2, 3), 2 => array(), 3 => array(4), 4 => null);
+        $b = array();
+        $expecteda = array(1 => array(2, 3, 4), 2 => array(), 3 => array(4), 4 => array());
+        $expectedb = array(1 => 1);
+
+        test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
+        $this->assertSame($expecteda, $a);
+        $this->assertSame($expectedb, $b);
+
+        // Also incorrect non-array dependency.
+        $a = array(1 => array(2, 3), 2 => array(), 3 => array(4), 4 => 23);
+        $b = array();
+        $expecteda = array(1 => array(2, 3, 4), 2 => array(), 3 => array(4), 4 => array());
+        $expectedb = array(1 => 1);
+
+        test_grade_grade_flatten_dependencies_array::test_flatten_dependencies_array($a, $b);
+        $this->assertSame($expecteda, $a);
+        $this->assertSame($expectedb, $b);
     }
 
     public function test_grade_grade_min_max() {