diff --git a/ext/uri/php_uri.c b/ext/uri/php_uri.c
index a09a93dbc0365..e5436d7f1e369 100644
--- a/ext/uri/php_uri.c
+++ b/ext/uri/php_uri.c
@@ -323,6 +323,10 @@ ZEND_ATTRIBUTE_NONNULL_ARGS(1, 2) PHPAPI void php_uri_instantiate_uri(
 	uri_object_t *uri_object;
 	if (should_update_this_object) {
 		uri_object = Z_URI_OBJECT_P(ZEND_THIS);
+		if (uri_object->internal.uri != NULL) {
+			zend_throw_error(NULL, "Cannot modify readonly object of class %s", ZSTR_VAL(Z_OBJCE_P(ZEND_THIS)->name));
+			RETURN_THROWS();
+		}
 	} else {
 		if (EX(func)->common.fn_flags & ZEND_ACC_STATIC) {
 			object_init_ex(return_value, Z_CE_P(ZEND_THIS));
@@ -842,7 +846,12 @@ static void uri_unserialize(INTERNAL_FUNCTION_PARAMETERS)
 	}
 
 	uri_internal_t *internal_uri = uri_internal_from_obj(object);
-	internal_uri->parser->free(internal_uri->uri);
+	if (internal_uri->uri != NULL) {
+		/* Intentionally throw two exceptions for proper chaining. */
+		zend_throw_error(NULL, "Cannot modify readonly object of class %s", ZSTR_VAL(Z_OBJCE_P(ZEND_THIS)->name));
+		zend_throw_exception_ex(NULL, 0, "Invalid serialization data for %s object", ZSTR_VAL(object->ce->name));
+		RETURN_THROWS();
+	}
 	internal_uri->uri = internal_uri->parser->parse(Z_STRVAL_P(uri_zv), Z_STRLEN_P(uri_zv), NULL, NULL, true);
 	if (internal_uri->uri == NULL) {
 		zend_throw_exception_ex(NULL, 0, "Invalid serialization data for %s object", ZSTR_VAL(object->ce->name));
diff --git a/ext/uri/tests/065.phpt b/ext/uri/tests/065.phpt
new file mode 100644
index 0000000000000..d9a6c905c9ef8
--- /dev/null
+++ b/ext/uri/tests/065.phpt
@@ -0,0 +1,117 @@
+--TEST--
+Test that overwriting the URI is not possible
+--EXTENSIONS--
+uri
+--FILE--
+<?php
+
+$uri = new Uri\WhatWg\Url('https://example.com');
+try {
+  $uri->__construct('ftp://example.org');
+} catch (Throwable $e) {
+  echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+}
+var_dump($uri);
+
+$uri = new Uri\WhatWg\Url('https://example.com');
+try {
+  $uri->__unserialize([['uri' => 'ftp://example.org'], []]);
+} catch (Throwable $e) {
+  echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+}
+var_dump($uri);
+
+$uri = new Uri\Rfc3986\Uri('https://example.com');
+try {
+  $uri->__construct('ftp://example.org');
+} catch (Throwable $e) {
+  echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+}
+var_dump($uri);
+
+$uri = new Uri\Rfc3986\Uri('https://example.com');
+try {
+  $uri->__unserialize([['uri' => 'ftp://example.org'], []]);
+} catch (Throwable $e) {
+  echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+}
+var_dump($uri);
+
+?>
+--EXPECTF--
+Error: Cannot modify readonly object of class Uri\WhatWg\Url
+object(Uri\WhatWg\Url)#%d (8) {
+  ["scheme"]=>
+  string(5) "https"
+  ["username"]=>
+  NULL
+  ["password"]=>
+  NULL
+  ["host"]=>
+  string(11) "example.com"
+  ["port"]=>
+  NULL
+  ["path"]=>
+  string(1) "/"
+  ["query"]=>
+  NULL
+  ["fragment"]=>
+  NULL
+}
+Exception: Invalid serialization data for Uri\WhatWg\Url object
+object(Uri\WhatWg\Url)#%d (8) {
+  ["scheme"]=>
+  string(5) "https"
+  ["username"]=>
+  NULL
+  ["password"]=>
+  NULL
+  ["host"]=>
+  string(11) "example.com"
+  ["port"]=>
+  NULL
+  ["path"]=>
+  string(1) "/"
+  ["query"]=>
+  NULL
+  ["fragment"]=>
+  NULL
+}
+Error: Cannot modify readonly object of class Uri\Rfc3986\Uri
+object(Uri\Rfc3986\Uri)#%d (8) {
+  ["scheme"]=>
+  string(5) "https"
+  ["username"]=>
+  NULL
+  ["password"]=>
+  NULL
+  ["host"]=>
+  string(11) "example.com"
+  ["port"]=>
+  NULL
+  ["path"]=>
+  string(0) ""
+  ["query"]=>
+  NULL
+  ["fragment"]=>
+  NULL
+}
+Exception: Invalid serialization data for Uri\Rfc3986\Uri object
+object(Uri\Rfc3986\Uri)#%d (8) {
+  ["scheme"]=>
+  string(5) "https"
+  ["username"]=>
+  NULL
+  ["password"]=>
+  NULL
+  ["host"]=>
+  string(11) "example.com"
+  ["port"]=>
+  NULL
+  ["path"]=>
+  string(0) ""
+  ["query"]=>
+  NULL
+  ["fragment"]=>
+  NULL
+}