Warning: mysqli_stmt::close() [mysqli-stmt.close]: invalid object or resource mysqli_stmt... why?

for learning purposes, why does php cries about the following code and gi开发者_运维技巧ves the following warning:

Warning: mysqli_stmt::close() [mysqli-stmt.close]: invalid object or resource mysqli_stmt in C:\public_html\admin\disctrack.php on line 14


12    $conn = connect('w'); // connect to a database in write mode
13    $stmt = $conn->stmt_init(); // initialize a prepared statement
14    $stmt->close(); // close statement

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From the mysqli::stmt_init documentation:

Any subsequent calls to any mysqli_stmt function will fail until mysqli_stmt_prepare() was called.