diff --git a/0-Plugin/src/main/java/main/Main.java b/0-Plugin/src/main/java/main/Main.java
index 2ad1c05..7e72624 100644
--- a/0-Plugin/src/main/java/main/Main.java
+++ b/0-Plugin/src/main/java/main/Main.java
@@ -7,9 +7,8 @@ public class Main {
 
 
 	public static void main(String[]args){
-
-		TagName a = new TagName("ds");
-		System.out.println("Schnauze");
+		TagName test = new TagName("ds");
+		System.out.println(test);
 	}
 
 }
diff --git a/3-Domain/src/main/java/link/Link.java b/3-Domain/src/main/java/link/Link.java
index 61c8bf9..29582ec 100644
--- a/3-Domain/src/main/java/link/Link.java
+++ b/3-Domain/src/main/java/link/Link.java
@@ -1,7 +1,7 @@
 package link;
 
 import category.CategoryId;
-import tag.TagMatcher;
+import tag.Tag;
 import user.Username;
 
 import java.util.Set;
@@ -12,9 +12,9 @@ public class Link {
 	private final Username creator;
 	private final LinkUrl url;
 	private final Set<CategoryId> categoryIds;
-	private final Set<TagMatcher> tags;
+	private final Set<Tag> tags;
 
-	public Link(LinkId id, Username creator, LinkUrl url, Set<CategoryId> categoryIds, Set<TagMatcher> tags) {
+	public Link(LinkId id, Username creator, LinkUrl url, Set<CategoryId> categoryIds, Set<Tag> tags) {
 		this.id = id;
 		this.creator = creator;
 		this.url = url;
diff --git a/3-Domain/src/main/java/tag/TagName.java b/3-Domain/src/main/java/tag/TagName.java
index edcc0ab..95cbf4d 100644
--- a/3-Domain/src/main/java/tag/TagName.java
+++ b/3-Domain/src/main/java/tag/TagName.java
@@ -1,5 +1,12 @@
 package tag;
 
+import exeptions.IllegalValueObjectArgument;
+
 public record TagName(String name) {
 
+	public TagName {
+		if (name.isEmpty() || name.isBlank()) {
+			throw new IllegalValueObjectArgument("A Tag name must be a valid non-empty string.");
+		}
+	}
 }
diff --git a/3-Domain/src/main/java/user/Username.java b/3-Domain/src/main/java/user/Username.java
index 304e467..27eb83c 100644
--- a/3-Domain/src/main/java/user/Username.java
+++ b/3-Domain/src/main/java/user/Username.java
@@ -1,5 +1,12 @@
 package user;
 
+import exeptions.IllegalValueObjectArgument;
+
 public record Username(String username) {
-	//TODO: checks if username is valid
+
+	public Username {
+		if (username.isBlank() || username.isEmpty()) {
+			throw new IllegalValueObjectArgument("A username must be a valid non-empty string.");
+		}
+	}
 }