diff --git a/oparl-server/src/main/java/de/twomartens/oparlservice/entity/File.java b/oparl-server/src/main/java/de/twomartens/oparlservice/entity/File.java
index 3bb468c..35b3663 100644
--- a/oparl-server/src/main/java/de/twomartens/oparlservice/entity/File.java
+++ b/oparl-server/src/main/java/de/twomartens/oparlservice/entity/File.java
@@ -41,6 +41,7 @@ public class File extends CommonFields {
     @Schema(description = "pure text representation of the file content if possible", nullable = true)
     private final String text;
 
+    @NonNull
     @Schema(description = "URL to access the file", required = true)
     private final String accessUrl;