diff --git a/types/xregexp/index.d.ts b/types/xregexp/index.d.ts
index 3974682fa0..35cd7cf970 100644
--- a/types/xregexp/index.d.ts
+++ b/types/xregexp/index.d.ts
@@ -84,7 +84,7 @@ declare namespace OuterXRegExp {
     function uninstall(options: Object): void;
     function uninstall(options: string): void;
 
-    function union(patterns: string[] | RegExp[], flags?: string): RegExp;
+    function union(patterns: (string | RegExp)[], flags?: string): RegExp;
     var version: string;
     // end API definitions
 
@@ -127,7 +127,7 @@ declare namespace OuterXRegExp {
         function uninstall(options: Object): void;
         function uninstall(options: string): void;
 
-        function union(patterns: string[] | RegExp[], flags?: string): RegExp;
+        function union(patterns: (string | RegExp)[], flags?: string): RegExp;
         var version: string;
         // end API definitions
     }
diff --git a/types/xregexp/xregexp-tests.ts b/types/xregexp/xregexp-tests.ts
index 543ceb7227..1dc60bce27 100644
--- a/types/xregexp/xregexp-tests.ts
+++ b/types/xregexp/xregexp-tests.ts
@@ -8,6 +8,7 @@ import TokenOpts = X.TokenOpts;
 var exp: RegExp;
 var expArr: RegExp[];
 var expArrArr: RegExp[][];
+var strOrExpArr: (string | RegExp)[];
 var chain: RegExp[];
 var groupChain: { regex: RegExp; backref: string }[];
 var groupChain1: { regex: RegExp; backref: number }[];
@@ -147,5 +148,7 @@ regex = XRegExp.union(strArr, flags);
 regex = XRegExp.union(strArr);
 regex = XRegExp.union(expArr, flags);
 regex = XRegExp.union(expArr);
+regex = XRegExp.union(strOrExpArr, flags);
+regex = XRegExp.union(strOrExpArr);
 
 // --  --  --  --  --  --  --  --  --  --  --  --  --