diff --git a/mod/assign/gradeform.php b/mod/assign/gradeform.php
index 2aab572bab6..598a1b51f85 100644
--- a/mod/assign/gradeform.php
+++ b/mod/assign/gradeform.php
@@ -89,7 +89,7 @@ class mod_assign_grade_form extends moodleform {
         }
 
         if ($instance->grade > 0) {
-            if (unformat_float($data['grade']) === null && (!empty($data['grade']))) {
+            if (!unformat_float($data['grade'], true) && (!empty($data['grade']))) {
                 $errors['grade'] = get_string('invalidfloatforgrade', 'assign', $data['grade']);
             } else if (unformat_float($data['grade']) > $instance->grade) {
                 $errors['grade'] = get_string('gradeabovemaximum', 'assign', $instance->grade);
diff --git a/mod/assign/tests/behat/display_error_message_onbadformat.feature b/mod/assign/tests/behat/display_error_message_onbadformat.feature
new file mode 100644
index 00000000000..d6db5ab2c8e
--- /dev/null
+++ b/mod/assign/tests/behat/display_error_message_onbadformat.feature
@@ -0,0 +1,66 @@
+@mod @mod_assign
+Feature: Check that the assignment grade can not be input in a wrong format.
+  In order to ensure that the grade is entered in the right format
+  As a teacher
+  I need to grade a student and ensure that the grade should be correctly entered
+
+  @javascript
+  Scenario: Error in the decimal separator ,
+    Given the following "courses" exist:
+      | fullname | shortname | category | groupmode |
+      | Course 1 | C1 | 0 | 1 |
+    And the following "users" exist:
+      | username | firstname | lastname | email |
+      | teacher1 | Teacher | 1 | teacher1@example.com |
+      | student1 | Student | 1 | student10@example.com |
+    And the following "course enrolments" exist:
+      | user | course | role |
+      | teacher1 | C1 | editingteacher |
+      | student1 | C1 | student |
+    And the following "groups" exist:
+      | name | course | idnumber |
+      | Group 1 | C1 | G1 |
+    And I log in as "teacher1"
+    And I follow "Course 1"
+    And I turn editing mode on
+    And I add a "Assignment" to section "1" and I fill the form with:
+      | Assignment name | Test assignment name |
+      | Description | Test assignment description |
+      | Use marking workflow | Yes |
+    When I follow "Test assignment name"
+    Then I follow "View/grade all submissions"
+    And I click on "Grade Student 1" "link" in the "Student 1" "table_row"
+    And I set the field "Grade out of 100" to "50,,6"
+    And I press "Save changes"
+    And I should see "The grade provided could not be understood: 50,,6"
+
+  @javascript
+  Scenario: Error in the decimal separator .
+    Given the following "courses" exist:
+      | fullname | shortname | category | groupmode |
+      | Course 1 | C1 | 0 | 1 |
+    And the following "users" exist:
+      | username | firstname | lastname | email |
+      | teacher1 | Teacher | 1 | teacher1@example.com |
+      | student1 | Student | 1 | student10@example.com |
+    And the following "course enrolments" exist:
+      | user | course | role |
+      | teacher1 | C1 | editingteacher |
+      | student1 | C1 | student |
+    And the following "groups" exist:
+      | name | course | idnumber |
+      | Group 1 | C1 | G1 |
+    And I log in as "teacher1"
+    And I follow "Course 1"
+    And I turn editing mode on
+    And I add a "Assignment" to section "1" and I fill the form with:
+      | Assignment name | Test assignment name |
+      | Description | Test assignment description |
+      | Use marking workflow | Yes |
+    When I follow "Test assignment name"
+    Then I follow "View/grade all submissions"
+    And I click on "Grade Student 1" "link" in the "Student 1" "table_row"
+    And I set the field "Grade out of 100" to "50..6"
+    And I press "Save changes"
+    And I should see "The grade provided could not be understood: 50..6"
+