diff --git a/src/main/java/de/hs_mannheim/domain/System.java b/src/main/java/de/hs_mannheim/domain/System.java
index 36ed716..e99da42 100644
--- a/src/main/java/de/hs_mannheim/domain/System.java
+++ b/src/main/java/de/hs_mannheim/domain/System.java
@@ -103,10 +103,22 @@ public class System {
     public boolean sign_up_user(String username, String password, String hometown, String zipS,
                                 String car_name, String car_l_100kmS, String car_avg_kmhS, String bike_avg_kmhS){
 
-        int zip = Integer.parseInt(zipS);
-        double car_l_100km = Double.parseDouble(car_l_100kmS);
-        double car_avg_kmh = Double.parseDouble(car_avg_kmhS);
-        double bike_avg_kmh = Double.parseDouble(bike_avg_kmhS);
+        int zip;
+        double car_l_100km;
+        double car_avg_kmh;
+        double bike_avg_kmh;
+    
+        try{
+            zip = Integer.parseInt(zipS);
+            car_l_100km = Double.parseDouble(car_l_100kmS);
+            car_avg_kmh = Double.parseDouble(car_avg_kmhS);
+            bike_avg_kmh = Double.parseDouble(bike_avg_kmhS);
+        } catch (NumberFormatException n){
+            return false;
+        }
+
+        if(username.equals("")||password.equals("")||hometown.equals("")||zipS.equals(""))
+            return false;
 
         for(User user: this.all_user)
             if(user.getUsername().equals(username))
diff --git a/src/main/java/de/hs_mannheim/domain/User.java b/src/main/java/de/hs_mannheim/domain/User.java
index a1d202e..b1f311b 100644
--- a/src/main/java/de/hs_mannheim/domain/User.java
+++ b/src/main/java/de/hs_mannheim/domain/User.java
@@ -5,11 +5,11 @@ public class User {
     private String username = "";
     private String password = "";
     private String hometown = "";
-    private int zip;
+    private int zip = 0;
     private String car_name = "";
-    private double car_l_100km;
-    private double car_avg_kmh;
-    private double bike_avg_kmh;
+    private double car_l_100km = 0;
+    private double car_avg_kmh = 0;
+    private double bike_avg_kmh = 0;
 
     public User(){}