diff --git a/src/UglyToad.PdfPig.Tests/TestPdfImage.cs b/src/UglyToad.PdfPig.Tests/TestPdfImage.cs
index b32e20fe..69a7212a 100644
--- a/src/UglyToad.PdfPig.Tests/TestPdfImage.cs
+++ b/src/UglyToad.PdfPig.Tests/TestPdfImage.cs
@@ -15,7 +15,7 @@
 
         public int HeightInSamples { get; set; }
 
-        public ColorSpace? ColorSpace => IsImageMask ? null : ColorSpaceDetails.Type;
+        public ColorSpace? ColorSpace => IsImageMask ? default(ColorSpace?) : ColorSpaceDetails.Type;
 
         public int BitsPerComponent { get; set; } = 8;