diff --git a/picard.wdl b/picard.wdl
index 136081be0e341ee2656299f5309868c867faea34..3103ad9baa7287a211a652e892dfa45a1071bebc 100644
--- a/picard.wdl
+++ b/picard.wdl
@@ -132,7 +132,7 @@ task CollectMultipleMetrics {
         File? qualityDistributionPdf = basename + ".quality_distribution.pdf"
         File? qualityYield = basename + ".quality_yield_metrics"
         # Using a glob is easier. But will lead to very ugly output directories.
-        Array[File]? allStats = select_all([
+        Array[File] allStats = select_all([
             alignmentSummary,
             baitBiasDetail,
             baitBiasSummary,