Index: src/wp-includes/capabilities.php
===================================================================
--- src/wp-includes/capabilities.php	(revision 39359)
+++ src/wp-includes/capabilities.php	(working copy)
@@ -429,11 +429,20 @@
 	case 'assign_term':
 		$term_id = $args[0];
 		$term = get_term( $term_id );
-		if ( ! $term || is_wp_error( $term ) ) {
+		if ( ! $term ) {
 			$caps[] = 'do_not_allow';
 			break;
 		}
 
+		if ( is_wp_error( $term ) ) {
+			if ( 'ambiguous_term_id' === $term->get_error_code() ) {
+				_split_shared_term( $term_id, /* $tt_id */ );
+			}
+
+			$caps[] = 'do_not_allow';
+			break;
+		}
+
 		$tax = get_taxonomy( $term->taxonomy );
 		if ( ! $tax ) {
 			$caps[] = 'do_not_allow';