Index: src/wp-admin/includes/upgrade.php
===================================================================
--- src/wp-admin/includes/upgrade.php	(revision 32367)
+++ src/wp-admin/includes/upgrade.php	(working copy)
@@ -1584,7 +1584,7 @@
 			$wpdb->query( "ALTER TABLE $wpdb->usermeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
 			$wpdb->query( "ALTER TABLE $wpdb->site DROP INDEX domain, ADD INDEX domain(domain(140),path(51))" );
 			$wpdb->query( "ALTER TABLE $wpdb->sitemeta DROP INDEX meta_key, ADD INDEX meta_key(meta_key(191))" );
-			$wpdb->query( "ALTER TABLE $wpdb->signups DROP INDEX domain, ADD INDEX domain(domain(140),path(51))" );
+			$wpdb->query( "ALTER TABLE $wpdb->signups DROP INDEX domain_path, ADD INDEX domain_path(domain(140),path(51))" );
 
 			$tables = $wpdb->tables( 'global' );
 
@@ -1593,6 +1593,25 @@
 			}
 		}
 	}
+
+	// 4.3
+	if ( $wp_current_db_version < 31374 && 'utf8mb4' === $wpdb->charset ) {
+		if ( ! ( defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) && DO_NOT_UPGRADE_GLOBAL_TABLES ) ) {
+			$upgrade = false;
+			$indexes = $wpdb->get_results( "SHOW INDEXES FROM $wpdb->signups" );
+			foreach( $indexes as $index ) {
+				if ( 'domain_path' == $index->Key_name && 'domain' == $index->Column_name && 140 != $index->Sub_part ) {
+					$upgrade = true;
+					break;
+				}
+			}
+
+			if ( $upgrade ) {
+				$wpdb->query( "ALTER TABLE $wpdb->signups DROP INDEX domain_path, ADD INDEX domain_path(domain(140),path(51))" );
+				maybe_convert_table_to_utf8mb4( $wpdb->signups );
+			}
+		}
+	}
 }
 
 //
Index: src/wp-includes/version.php
===================================================================
--- src/wp-includes/version.php	(revision 32367)
+++ src/wp-includes/version.php	(working copy)
@@ -11,7 +11,7 @@
  *
  * @global int $wp_db_version
  */
-$wp_db_version = 32364;
+$wp_db_version = 32374;
 
 /**
  * Holds the TinyMCE version