diff --git a/src/wp-includes/IXR/class-IXR-client.php b/src/wp-includes/IXR/class-IXR-client.php
index 34c1f9337c461..3fa37576c2b61 100644
--- a/src/wp-includes/IXR/class-IXR-client.php
+++ b/src/wp-includes/IXR/class-IXR-client.php
@@ -30,7 +30,7 @@ function __construct( $server, $path = false, $port = 80, $timeout = 15 )
         if (!$path) {
             // Assume we have been given a URL instead
             $bits = parse_url($server);
-            $this->server = $bits['host'];
+            $this->server = $bits['host'] ?? '';
             $this->port = $bits['port'] ?? 80;
             $this->path = $bits['path'] ?? '/';
 
diff --git a/src/wp-includes/class-wp-http-ixr-client.php b/src/wp-includes/class-wp-http-ixr-client.php
index 71875d550fc2f..f8b0b144f8b26 100644
--- a/src/wp-includes/class-wp-http-ixr-client.php
+++ b/src/wp-includes/class-wp-http-ixr-client.php
@@ -23,10 +23,10 @@ public function __construct( $server, $path = false, $port = false, $timeout = 1
 		if ( ! $path ) {
 			// Assume we have been given a URL instead.
 			$bits         = parse_url( $server );
-			$this->scheme = $bits['scheme'];
-			$this->server = $bits['host'];
+			$this->scheme = $bits['scheme'] ?? '';
+			$this->server = $bits['host'] ?? '';
 			$this->port   = $bits['port'] ?? $port;
-			$this->path   = ! empty( $bits['path'] ) ? $bits['path'] : '/';
+			$this->path   = $bits['path'] ?? '/';
 
 			// Make absolutely sure we have a path.
 			if ( ! $this->path ) {
diff --git a/tests/phpunit/tests/xmlrpc/client.php b/tests/phpunit/tests/xmlrpc/client.php
index a13980c17d174..75734d8e150b7 100644
--- a/tests/phpunit/tests/xmlrpc/client.php
+++ b/tests/phpunit/tests/xmlrpc/client.php
@@ -17,6 +17,14 @@ public function test_ixr_client_allows_query_strings() {
 		$this->assertSame( '/server.php?this-is-needed=true', $client->path );
 	}
 
+	/**
+	 * @ticket 64635
+	 */
+	public function test_ixr_client_can_handle_missing_host() {
+		$client = new IXR_Client( '/no-host-here' );
+		$this->assertSame( '', $client->server );
+	}
+
 	/**
 	 * @ticket 26947
 	 */
@@ -26,4 +34,33 @@ public function test_wp_ixr_client_allows_query_strings() {
 		$this->assertFalse( $client->port );
 		$this->assertSame( '/server.php?this-is-needed=true', $client->path );
 	}
+
+	/**
+	 * @ticket 40784
+	 */
+	public function test_wp_ixr_client_can_handle_protocolless_urls() {
+		$client = new WP_HTTP_IXR_Client( '//example.com/server.php' );
+		$this->assertSame( '', $client->scheme );
+		$this->assertSame( 'example.com', $client->server );
+	}
+
+	/**
+	 * @ticket 40784
+	 */
+	public function test_wp_ixr_client_can_handle_relative_urls() {
+		$client = new WP_HTTP_IXR_Client( '/server.php' );
+		$this->assertSame( '', $client->scheme );
+		$this->assertSame( '', $client->server );
+		$this->assertSame( '/server.php', $client->path );
+	}
+
+	/**
+	 * @ticket 40784
+	 */
+	public function test_wp_ixr_client_can_handle_invalid_urls() {
+		$client = new WP_HTTP_IXR_Client( '' );
+		$this->assertSame( '', $client->scheme );
+		$this->assertSame( '', $client->server );
+		$this->assertSame( '/', $client->path );
+	}
 }