diff --git a/wp-includes/class-wp-block-styles-registry.php b/wp-includes/class-wp-block-styles-registry.php
index 67d6998fc5..272e400f8c 100644
--- a/wp-includes/class-wp-block-styles-registry.php
+++ b/wp-includes/class-wp-block-styles-registry.php
@@ -62,6 +62,15 @@ final class WP_Block_Styles_Registry {
 			return false;
 		}
 
+		if ( str_contains( $style_properties['name'], ' ' ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Block style name must not contain any spaces.' ),
+				'5.9.0'
+			);
+			return false;
+		}
+
 		$block_style_name = $style_properties['name'];
 
 		if ( ! isset( $this->registered_block_styles[ $block_name ] ) ) {
diff --git a/wp-includes/version.php b/wp-includes/version.php
index dcec740d67..1cf6a10481 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-52260';
+$wp_version = '5.9-alpha-52261';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.