diff --git a/grade/classes/component_gradeitem.php b/grade/classes/component_gradeitem.php
index 060e5b89776..8aea26bf02a 100644
--- a/grade/classes/component_gradeitem.php
+++ b/grade/classes/component_gradeitem.php
@@ -405,6 +405,7 @@ abstract class component_gradeitem {
         if ($grade = $this->get_grade_for_user($gradeduser, $grader)) {
             $gradeitem = $this->get_grade_item();
             if (!$this->is_using_scale()) {
+                $grade->grade = !is_null($grade->grade) ? (float)$grade->grade : null; // Cast non-null values, keeping nulls.
                 $grade->usergrade = grade_format_gradevalue($grade->grade, $gradeitem);
                 $grade->maxgrade = format_float($gradeitem->grademax, $gradeitem->get_decimals());
                 // If displaying the raw grade, also display the total value.
diff --git a/lib/gradelib.php b/lib/gradelib.php
index daea2b4754f..91c3f680c61 100644
--- a/lib/gradelib.php
+++ b/lib/gradelib.php
@@ -759,14 +759,14 @@ function grade_set_setting($courseid, $name, $value) {
 /**
  * Returns string representation of grade value
  *
- * @param float $value The grade value
+ * @param float|null $value The grade value
  * @param object $grade_item Grade item object passed by reference to prevent scale reloading
  * @param bool $localized use localised decimal separator
  * @param int $displaytype type of display. For example GRADE_DISPLAY_TYPE_REAL, GRADE_DISPLAY_TYPE_PERCENTAGE, GRADE_DISPLAY_TYPE_LETTER
  * @param int $decimals The number of decimal places when displaying float values
  * @return string
  */
-function grade_format_gradevalue($value, &$grade_item, $localized=true, $displaytype=null, $decimals=null) {
+function grade_format_gradevalue(?float $value, &$grade_item, $localized=true, $displaytype=null, $decimals=null) {
     if ($grade_item->gradetype == GRADE_TYPE_NONE or $grade_item->gradetype == GRADE_TYPE_TEXT) {
         return '';
     }
@@ -830,13 +830,13 @@ function grade_format_gradevalue($value, &$grade_item, $localized=true, $display
 /**
  * Returns a float representation of a grade value
  *
- * @param float $value The grade value
+ * @param float|null $value The grade value
  * @param object $grade_item Grade item object
  * @param int $decimals The number of decimal places
  * @param bool $localized use localised decimal separator
  * @return string
  */
-function grade_format_gradevalue_real($value, $grade_item, $decimals, $localized) {
+function grade_format_gradevalue_real(?float $value, $grade_item, $decimals, $localized) {
     if ($grade_item->gradetype == GRADE_TYPE_SCALE) {
         if (!$scale = $grade_item->load_scale()) {
             return get_string('error');
@@ -853,13 +853,13 @@ function grade_format_gradevalue_real($value, $grade_item, $decimals, $localized
 /**
  * Returns a percentage representation of a grade value
  *
- * @param float $value The grade value
+ * @param float|null $value The grade value
  * @param object $grade_item Grade item object
  * @param int $decimals The number of decimal places
  * @param bool $localized use localised decimal separator
  * @return string
  */
-function grade_format_gradevalue_percentage($value, $grade_item, $decimals, $localized) {
+function grade_format_gradevalue_percentage(?float $value, $grade_item, $decimals, $localized) {
     $min = $grade_item->grademin;
     $max = $grade_item->grademax;
     if ($min == $max) {
@@ -874,11 +874,11 @@ function grade_format_gradevalue_percentage($value, $grade_item, $decimals, $loc
  * Returns a letter grade representation of a grade value
  * The array of grade letters used is produced by {@link grade_get_letters()} using the course context
  *
- * @param float $value The grade value
+ * @param float|null $value The grade value
  * @param object $grade_item Grade item object
  * @return string
  */
-function grade_format_gradevalue_letter($value, $grade_item) {
+function grade_format_gradevalue_letter(?float $value, $grade_item) {
     global $CFG;
     $context = context_course::instance($grade_item->courseid, IGNORE_MISSING);
     if (!$letters = grade_get_letters($context)) {
@@ -1587,10 +1587,10 @@ function grade_course_reset($courseid) {
  * Convert a number to 5 decimal point float, an empty string or a null db compatible format
  * (we need this to decide if db value changed)
  *
- * @param mixed $number The number to convert
- * @return mixed float or null
+ * @param float|null $number The number to convert
+ * @return float|null float or null
  */
-function grade_floatval($number) {
+function grade_floatval(?float $number) {
     if (is_null($number) or $number === '') {
         return null;
     }
@@ -1603,11 +1603,11 @@ function grade_floatval($number) {
  * Compare two float numbers safely. Uses 5 decimals php precision using {@link grade_floatval()}. Nulls accepted too.
  * Used for determining if a database update is required
  *
- * @param float $f1 Float one to compare
- * @param float $f2 Float two to compare
+ * @param float|null $f1 Float one to compare
+ * @param float|null $f2 Float two to compare
  * @return bool True if the supplied values are different
  */
-function grade_floats_different($f1, $f2) {
+function grade_floats_different(?float $f1, ?float $f2): bool {
     // note: db rounding for 10,5 is different from php round() function
     return (grade_floatval($f1) !== grade_floatval($f2));
 }
@@ -1619,11 +1619,11 @@ function grade_floats_different($f1, $f2) {
  * different from php round() function.
  *
  * @since Moodle 2.0
- * @param float $f1 Float one to compare
- * @param float $f2 Float two to compare
+ * @param float|null $f1 Float one to compare
+ * @param float|null $f2 Float two to compare
  * @return bool True if the values should be considered as the same grades
  */
-function grade_floats_equal($f1, $f2) {
+function grade_floats_equal(?float $f1, ?float $f2): bool {
     return (grade_floatval($f1) === grade_floatval($f2));
 }
 
diff --git a/lib/upgrade.txt b/lib/upgrade.txt
index fbdce39d287..70050362a18 100644
--- a/lib/upgrade.txt
+++ b/lib/upgrade.txt
@@ -2,6 +2,16 @@ This files describes API changes in core libraries and APIs,
 information provided here is intended especially for developers.
 
 === 4.0 ===
+* To better detect wrong floats (like, for example, unformatted, using local-dependent separators ones) a number of
+  gradebook functions now have stricter float type checking. All them will require now the "float" being passed to be
+  a correct float value (numeric or string). Usually, that's achieved by using unformat_float() or
+  PARAM_LOCALISEDFLOAT for all the user-entered grades before any processing on them. Functions affected are:
+    - grade_format_gradevalue(), $value param (keeping it as optional/nullable).
+    - grade_format_gradevalue_real(), $value param (keeping it as optional/nullable).
+    - grade_format_gradevalue_percentage(), $value param (keeping it as optional/nullable).
+    - grade_format_gradevalue_letter(), $value param (keeping it as optional/nullable).
+    - grade_floats_different(), $f1 and $f2 params (keeping them as optional/nullable).
+    - grade_floats_equal(), $f1 and $f2 params (keeping them as optional/nullable).
 * The method action_menu->set_alignment() has been deprecated, please use action_menu->set_menu_left if you need a dropdown
   to align to the left of the dropdown button.
 * The $OUTPUT->should_display_main_logo() function has been deprecated and should no longer be used.