diff --git a/bwa.wdl b/bwa.wdl
index e97ae481fc592b2ecdfb91c9f53049de32d7530f..440b6559b2fcceaa0efaebfdeddd35b46da07d5a 100644
--- a/bwa.wdl
+++ b/bwa.wdl
@@ -2,7 +2,7 @@ task mem {
     String? preCommand
     File inputR1
     File? inputR2
-    String referenceFasta
+    File referenceFasta
     Array[File] indexFiles # These indexFiles need to be added, otherwise cromwell will not find them.
     String outputPath
     String? readgroup