Index: wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- wp-includes/class-wp-xmlrpc-server.php	(revision 22056)
+++ wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -1561,7 +1561,7 @@
 		if ( empty( $term_data['name'] ) )
 			return new IXR_Error( 403, __( 'The term name cannot be empty.' ) );
 
-		if ( isset( $content_struct['parent'] ) ) {
+		if ( ! empty( $content_struct['parent'] ) ) {
 			if ( ! $taxonomy['hierarchical'] )
 				return new IXR_Error( 403, __( 'This taxonomy is not hierarchical.' ) );
 
@@ -1660,7 +1660,7 @@
 				return new IXR_Error( 403, __( 'The term name cannot be empty.' ) );
 		}
 
-		if ( isset( $content_struct['parent'] ) ) {
+		if ( ! empty( $content_struct['parent'] ) ) {
 			if ( ! $taxonomy['hierarchical'] )
 				return new IXR_Error( 403, __( "This taxonomy is not hierarchical so you can't set a parent." ) );