diff --git a/README.MD b/README.MD
index 150ee94..f45dede 100644
--- a/README.MD
+++ b/README.MD
@@ -2,7 +2,7 @@
 =========
 
 ## About
-*Labyrinth* is a web api to navigate through a maze.
+*Labyrinth* is a web API to navigate through a maze.
 
 
 # How to run

diff --git a/README.MD b/README.MD
index 150ee94..f45dede 100644
--- a/README.MD
+++ b/README.MD
@@ -2,7 +2,7 @@
 =========
 
 ## About
-*Labyrinth* is a web api to navigate through a maze.
+*Labyrinth* is a web API to navigate through a maze.
 
 
 # How to run
diff --git a/package.json b/package.json
index 690e23f..7d1c926 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "labyrinth",
   "version": "0.1.0",
-  "description": "Labyrinth =========",
+  "description": "Labyrinth is a web API to navigate through a maze.",
   "main": "app.js",
   "scripts": {
     "test": "tsc && ./node_modules/mocha/bin/mocha dist/**/*.spec.js"

diff --git a/README.MD b/README.MD
index 150ee94..f45dede 100644
--- a/README.MD
+++ b/README.MD
@@ -2,7 +2,7 @@
 =========
 
 ## About
-*Labyrinth* is a web api to navigate through a maze.
+*Labyrinth* is a web API to navigate through a maze.
 
 
 # How to run
diff --git a/package.json b/package.json
index 690e23f..7d1c926 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "labyrinth",
   "version": "0.1.0",
-  "description": "Labyrinth =========",
+  "description": "Labyrinth is a web API to navigate through a maze.",
   "main": "app.js",
   "scripts": {
     "test": "tsc && ./node_modules/mocha/bin/mocha dist/**/*.spec.js"
diff --git a/src/GameServer.spec.ts b/src/GameServer.spec.ts
index 76c8775..743c0ff 100644
--- a/src/GameServer.spec.ts
+++ b/src/GameServer.spec.ts
@@ -58,12 +58,52 @@
             expect(position).to.be.null;
         });
     });
+
+    describe("When userID is null", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition(null);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When userID is undefined", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition(undefined);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When userID is an empty string", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition("");
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
+
+    describe("When userID is a whitespace string", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition("  ");
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
 });
 
 describe("Scenario: Starting the game", function(){
     const userID = "UserID";
     const mapID = "lvl1";
-    describe("when a user starts an existing valid level", function(){
+    describe("When a user starts an existing valid level", function(){
         const games = createGameServer();
         const position = games.startGame(userID, mapID);
 
@@ -76,6 +116,46 @@
             expect(current.id).to.equal(position.id);
         });
     });
+
+    describe("When the supplied userID is null", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame(null, mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When the supplied userID is undefined", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame(undefined, mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When the supplied userID is an empty string", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame("", mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
+
+    describe("When the supplied userID has only white spaces", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame("  ", mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
 });
 
 describe("Scenario: Navigating in ongoing game", function(){

diff --git a/README.MD b/README.MD
index 150ee94..f45dede 100644
--- a/README.MD
+++ b/README.MD
@@ -2,7 +2,7 @@
 =========
 
 ## About
-*Labyrinth* is a web api to navigate through a maze.
+*Labyrinth* is a web API to navigate through a maze.
 
 
 # How to run
diff --git a/package.json b/package.json
index 690e23f..7d1c926 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "labyrinth",
   "version": "0.1.0",
-  "description": "Labyrinth =========",
+  "description": "Labyrinth is a web API to navigate through a maze.",
   "main": "app.js",
   "scripts": {
     "test": "tsc && ./node_modules/mocha/bin/mocha dist/**/*.spec.js"
diff --git a/src/GameServer.spec.ts b/src/GameServer.spec.ts
index 76c8775..743c0ff 100644
--- a/src/GameServer.spec.ts
+++ b/src/GameServer.spec.ts
@@ -58,12 +58,52 @@
             expect(position).to.be.null;
         });
     });
+
+    describe("When userID is null", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition(null);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When userID is undefined", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition(undefined);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When userID is an empty string", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition("");
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
+
+    describe("When userID is a whitespace string", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.currentPosition("  ");
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
 });
 
 describe("Scenario: Starting the game", function(){
     const userID = "UserID";
     const mapID = "lvl1";
-    describe("when a user starts an existing valid level", function(){
+    describe("When a user starts an existing valid level", function(){
         const games = createGameServer();
         const position = games.startGame(userID, mapID);
 
@@ -76,6 +116,46 @@
             expect(current.id).to.equal(position.id);
         });
     });
+
+    describe("When the supplied userID is null", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame(null, mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When the supplied userID is undefined", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame(undefined, mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*/i);
+        });
+    });
+
+    describe("When the supplied userID is an empty string", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame("", mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
+
+    describe("When the supplied userID has only white spaces", function(){
+        const games = createGameServer();
+        it("then it should throw an exception", function(){
+            function expectedToFail(){
+                games.startGame("  ", mapID);
+            }
+            expect(expectedToFail).to.throw(/.*userid.*empty.*/i);
+        });
+    });
 });
 
 describe("Scenario: Navigating in ongoing game", function(){
diff --git a/src/GameServer.ts b/src/GameServer.ts
index 04a0818..5884e9d 100644
--- a/src/GameServer.ts
+++ b/src/GameServer.ts
@@ -55,6 +55,7 @@
 
     public startGame(userID: string, mapID: string): MazeTile
     {
+        this.userIdGuard(userID);
         const map = this.mapList.find(m => m.id === mapID);
         if(map == null){
             throw new Error(`Map "${mapID}" does not exist.`);
@@ -78,6 +79,8 @@
 
     public navigate(userID: string, direction:Direction): MazeTile
     {
+        this.userIdGuard(userID);
+
         const state = this.states.find(s => s.userID === userID);
         if(state ==  null){
             return null;
@@ -109,6 +112,8 @@
 
     public currentPosition(userID: string): MazeTile
     {
+        this.userIdGuard(userID);
+
         const state = this.states.find(s => s.userID === userID);
         if(state == null){
             return null;
@@ -118,4 +123,19 @@
         const tile = map.getTile(state.tileID);
         return tile;
     }
+
+    /**
+     * Makes sure, that a userID is acceptable.
+     * If not throw the appropriate error.
+     * @param userID supplied userID
+     */
+    private userIdGuard(userID: string): void
+    {
+        if(userID === undefined || userID === null){
+            throw new Error("UserID must be supplied!");
+        }
+        if(userID === "" || /^\s+$/.test(userID)){
+            throw new Error("UserID must not be empty.");
+        }
+    }
 }