diff --git a/src/InternshipSystem.Api/Controllers/DocumentsController.cs b/src/InternshipSystem.Api/Controllers/DocumentsController.cs
index 7cda5aa..bb124db 100644
--- a/src/InternshipSystem.Api/Controllers/DocumentsController.cs
+++ b/src/InternshipSystem.Api/Controllers/DocumentsController.cs
@@ -1,4 +1,5 @@
-using System.IO;
+using System;
+using System.IO;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -63,8 +64,15 @@ namespace InternshipSystem.Api.Controllers
                     .Include(i => i.Documentation)
                     .FirstAsync(i => i.Student.Id == user.PersonNumber, cancellationToken);
 
-            internship.AddNewDocument(documentRequest.Description, documentRequest.Type);
-            await _context.SaveChangesAsync(cancellationToken);
+            try
+            {
+                internship.AddNewDocument(documentRequest.Description, documentRequest.Type);
+                await _context.SaveChangesAsync(cancellationToken);
+            }
+            catch (ArgumentException e)
+            {
+                return BadRequest(e.Message);
+            }
 
             return Ok();
         }
diff --git a/src/InternshipSystem.Api/Controllers/EditionController.cs b/src/InternshipSystem.Api/Controllers/EditionController.cs
index 40834d4..35a4bc9 100644
--- a/src/InternshipSystem.Api/Controllers/EditionController.cs
+++ b/src/InternshipSystem.Api/Controllers/EditionController.cs
@@ -72,7 +72,7 @@ namespace InternshipSystem.Api.Controllers
             var edition = 
                 await Context.Editions
                     .Where(e => e.Id == id)
-                    .ProjectTo<EditionResult?>(Mapper.ConfigurationProvider)
+                    .ProjectTo<EditionResult>(Mapper.ConfigurationProvider)
                     .FirstOrDefaultAsync(token);
             
             if (edition == null) 
diff --git a/src/InternshipSystem.Api/Result/EditionResult.cs b/src/InternshipSystem.Api/Result/EditionResult.cs
index 5e91432..73b2a58 100644
--- a/src/InternshipSystem.Api/Result/EditionResult.cs
+++ b/src/InternshipSystem.Api/Result/EditionResult.cs
@@ -2,7 +2,7 @@
 
 namespace InternshipSystem.Api.Result
 {
-    public struct EditionResult
+    public class EditionResult
     {
         public Guid Id { get; set; }
         public DateTime EditionStart { get; set; }
diff --git a/src/InternshipSystem.Core/Entity/Internship/Internship.cs b/src/InternshipSystem.Core/Entity/Internship/Internship.cs
index 3d148f4..c2104e6 100644
--- a/src/InternshipSystem.Core/Entity/Internship/Internship.cs
+++ b/src/InternshipSystem.Core/Entity/Internship/Internship.cs
@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using InternshipSystem.Core.ValueObject;
 
@@ -31,6 +32,11 @@ namespace InternshipSystem.Core.Entity.Internship
 
         public void AddNewDocument(string description, DocumentType type)
         {
+            if (Documentation.Any(d => d.Type == type))
+            {
+                throw new ArgumentException("Internship already has a document of given type");                
+            }
+            
             var document = new Document
             {
                 Description = description,