diff --git a/samplesheet.wdl b/samplesheet.wdl
index e269911ddac6c9c353dcdcee8d95a8abf90713f6..4bec92cb737e237421fd28eef2bac04e1abf16c2 100644
--- a/samplesheet.wdl
+++ b/samplesheet.wdl
@@ -4,7 +4,7 @@ struct Readgroup {
     File R1
     String R1_md5
     File? R2
-    String R2_md5
+    String? R2_md5
 }
 
 struct Library {
@@ -70,4 +70,4 @@ task sampleConfigFileToStruct {
         Map[String,Array[Sample]] map = read_json(outputJson)
         Array[Sample] samples = map["samples"]
      }
-}
\ No newline at end of file
+}