php - Call to a member function bind_param() on boolean

I try to bind parameters to sql (mysqli) in php, but there is an error code as written above.
This is the code I wrote:

$stmt = $conn->prepare("INSERT INTO assignments (Classes_has_Subjects_classesHasSubjectsId, assignmentName, gradeForAssignment,protectiveGrade) 
                                  VALUES (?, ?, ?, ?)");
if ($stmt = false)

{
    echo "false";
}
else{
    $stmt->bind_param("sss", $classesHasSubjectsId, $assignmentName, $gradeForAssignment,  $protectiveGrade);
    $stmt->execute();
}

and here is the error message:

11

Fatal error: Call to a member function bind_param() on boolean in

What's wrong?