1 files changed, 1 insertions, 1 deletions

diff --git a/src/WixToolset.Core/PreprocessResult.cs b/src/WixToolset.Core/PreprocessResult.cs
index 7126f049..83b29a90 100644
--- a/src/WixToolset.Core/PreprocessResult.cs
+++ b/src/WixToolset.Core/PreprocessResult.cs
@@ -10,6 +10,6 @@ namespace WixToolset.Core
     {
         public XDocument Document { get; set; }
 
-        public IEnumerable<IIncludedFile> IncludedFiles { get; set; }
+        public IReadOnlyCollection<IIncludedFile> IncludedFiles { get; set; }
     }
 }