diff --git a/includes/engine/class-tmdo-schema-manager.php b/includes/engine/class-tmdo-schema-manager.php
index 527c9da..710f06b 100644
--- a/includes/engine/class-tmdo-schema-manager.php
+++ b/includes/engine/class-tmdo-schema-manager.php
@@ -231,6 +231,11 @@ final class TMDO_Schema_Manager {
 		// processes both "acquire" an option lock; GET_LOCK is atomic and is
 		// released by MySQL if this process dies.
 		$lock = self::index_lock_name();
+		if ( method_exists( $wpdb, 'send_reads_to_masters' ) ) {
+			// HyperDB / LudicrousDB would send the SELECT GET_LOCK to a replica,
+			// where it locks nothing against the ALTER on the primary.
+			$wpdb->send_reads_to_masters();
+		}
 		if ( '1' !== (string) $wpdb->get_var( $wpdb->prepare( 'SELECT GET_LOCK(MD5(CONCAT(DATABASE(), %s)), 0)', $lock ) ) ) {
 			return; // Another process is adding indexes.
 		}
diff --git a/tests/integration/GroupCompositeIndexTest.php b/tests/integration/GroupCompositeIndexTest.php
index e8d6e3e..4f335da 100644
--- a/tests/integration/GroupCompositeIndexTest.php
+++ b/tests/integration/GroupCompositeIndexTest.php
@@ -134,6 +134,45 @@ class GroupCompositeIndexTest extends TestCase {
 		$this->assertSame( '1', (string) $wpdb->get_var( $wpdb->prepare( 'SELECT IS_FREE_LOCK(MD5(CONCAT(DATABASE(), %s)))', TMDO_Schema_Manager::index_lock_name() ) ), 'lock released' );
 	}
 
+	/**
+	 * Under read/write splitting (HyperDB, LudicrousDB) a SELECT may go to a
+	 * replica, where GET_LOCK would lock nothing: reads go to the primary first.
+	 */
+	public function test_the_lock_is_taken_on_the_primary_under_read_write_splitting(): void {
+		global $wpdb;
+		$real = $wpdb;
+		$wpdb = new class( $real ) {
+			/** @var string[] */
+			public array $calls = array();
+
+			public function __construct( private object $db ) {}
+
+			public function send_reads_to_masters(): void {
+				$this->calls[] = 'send_reads_to_masters';
+			}
+
+			public function __call( string $name, array $args ) {
+				if ( 'get_var' === $name && str_contains( (string) $args[0], 'GET_LOCK' ) ) {
+					$this->calls[] = 'GET_LOCK';
+				}
+				return $this->db->$name( ...$args );
+			}
+
+			public function __get( string $name ) {
+				return $this->db->$name;
+			}
+		};
+		try {
+			TMDO_Schema_Manager::ensure_group_indexes( 'post', 'ci_core' );
+			$calls = $wpdb->calls;
+		} finally {
+			$wpdb = $real;
+		}
+
+		$this->assertSame( array( 'send_reads_to_masters', 'GET_LOCK' ), $calls );
+		$this->assertArrayHasKey( 'cidx_flag_price', $this->indexes() );
+	}
+
 	/** @return array<string, string[]> index name → ordered columns */
 	private function indexes(): array {
 		global $wpdb;