diff --git a/stringtie.wdl b/stringtie.wdl
index e969ac79e5b91cb69350d92f741c72936b6f1f5e..33118d3ed7a3384658e160c7a846bc2547fae41c 100644
--- a/stringtie.wdl
+++ b/stringtie.wdl
@@ -24,7 +24,7 @@ task Stringtie {
 
     output {
         File assembledTranscripts = assembledTranscriptsFile
-        File geneAbundance = geneAbundanceFile
+        File? geneAbundance = geneAbundanceFile
     }
 
     runtime {