diff --git a/picard.wdl b/picard.wdl
index 1d8376ca34a11c7de2c5fc1b2884f4c22486b4b2..0e877f23db9bea891e70225bf24c66368d8a76c1 100644
--- a/picard.wdl
+++ b/picard.wdl
@@ -388,7 +388,7 @@ task GatherBamFiles {
     output {
         File outputBam = outputBamPath
         File outputBamIndex = sub(outputBamPath, "\.bam$", ".bai")
-        File outputBamMd5 = outputBamPath + ".md5"
+        File? outputBamMd5 = outputBamPath + ".md5"
     }
 
     runtime {