diff --git a/Interfaces/ZUGFeRDInterface/Exceptions.vb b/Interfaces/ZUGFeRDInterface/Exceptions.vb
index 79af3acd..18e33155 100644
--- a/Interfaces/ZUGFeRDInterface/Exceptions.vb
+++ b/Interfaces/ZUGFeRDInterface/Exceptions.vb
@@ -54,7 +54,7 @@ Public Class Exceptions
 
         Public ValidationErrors As List(Of ZugferdValidationError)
 
-        Public Property ErrorCode() As ErrorCodes
+        Public ReadOnly Property ErrorCode() As ErrorCodes
 
         Public Sub New()
             MyBase.New("ZUGFeRD document found but validation failed!")