diff --git a/src/main/java/picard/analysis/MergeableMetricBase.java b/src/main/java/picard/analysis/MergeableMetricBase.java
index c4ebb2954..d729f90b9 100644
--- a/src/main/java/picard/analysis/MergeableMetricBase.java
+++ b/src/main/java/picard/analysis/MergeableMetricBase.java
@@ -125,7 +125,7 @@ public boolean mergeIfCan(final MergeableMetricBase other) {
      * @param many a Collection of MergeableMetricBase
      */
     public MergeableMetricBase merge(final Collection<? extends MergeableMetricBase> many) {
-        many.parallelStream().forEach(this::merge);
+        many.stream().forEach(this::merge);
         calculateDerivedFields();
         return this;
     }