diff --git a/src/language/printer.js b/src/language/printer.js
index 8665fee20b5..ade5cdcce68 100644
--- a/src/language/printer.js
+++ b/src/language/printer.js
@@ -184,7 +184,7 @@ const printDocASTReducer: any = {
         'union',
         name,
         join(directives, ' '),
-        types && types.length !== 0 ? '= ' + join(types, ' | ') : '',
+        wrap('= ', join(types, ' | ')),
       ],
       ' ',
     ),
@@ -253,7 +253,7 @@ const printDocASTReducer: any = {
         'extend union',
         name,
         join(directives, ' '),
-        types && types.length !== 0 ? '= ' + join(types, ' | ') : '',
+        wrap('= ', join(types, ' | ')),
       ],
       ' ',
     ),