diff --git a/types/mssql/index.d.ts b/types/mssql/index.d.ts
index 27afb14bcfecee..55db06e04b91a3 100644
--- a/types/mssql/index.d.ts
+++ b/types/mssql/index.d.ts
@@ -236,8 +236,7 @@ export declare class ConnectionPool extends events.EventEmitter {
     public static parseConnectionString(
         connectionString: string,
     ): config & { options: IOptions; pool: Partial<PoolOpts<Connection>> };
-    public constructor(config: config, callback?: (err?: any) => void);
-    public constructor(connectionString: string, callback?: (err?: any) => void);
+    public constructor(configOrConnectionString: config | string, callback?: (err?: any) => void);
     public query(command: string): Promise<IResult<any>>;
     public query(strings: TemplateStringsArray, ...interpolations: any[]): Promise<IResult<any>>;
     public query<Entity>(command: string): Promise<IResult<Entity>>;
diff --git a/types/mssql/mssql-tests.ts b/types/mssql/mssql-tests.ts
index 47593af36ef9fa..73944cd0087b73 100644
--- a/types/mssql/mssql-tests.ts
+++ b/types/mssql/mssql-tests.ts
@@ -36,6 +36,8 @@ var config: sql.config = {
 
 var connectionString = "Server=localhost,1433;Database=database;User Id=username;Password=password;Encrypt=true";
 
+var configOrConnectionString: sql.config | string;
+
 var minimalConfig: sql.config = { server: "ip" };
 
 var connectionStringTest: sql.ConnectionPool = new sql.ConnectionPool("connectionstring", (err) => {
@@ -163,6 +165,8 @@ var connection: sql.ConnectionPool = new sql.ConnectionPool(config, function(err
     }
 });
 
+var connectionUnionTypeTest = new sql.ConnectionPool(configOrConnectionString);
+
 function test_connection_string_parser() {
     var parsedConfig: sql.config = sql.ConnectionPool.parseConnectionString(connectionString);
     parsedConfig.pool; // $ExpectType PoolOpts<Connection>