diff --git a/src/main/java/gov/usgs/earthquake/nshmp/data/Sequences.java b/src/main/java/gov/usgs/earthquake/nshmp/data/Sequences.java
index e9eae04f41ab3acfc689a151e282d54d34c6b486..bc3c896de1f70436a5a66a0bf4f5828b501bcee1 100644
--- a/src/main/java/gov/usgs/earthquake/nshmp/data/Sequences.java
+++ b/src/main/java/gov/usgs/earthquake/nshmp/data/Sequences.java
@@ -104,7 +104,7 @@ class Sequences {
         .collect(Collectors.toMap(
             XyPoint::x,
             (XyPoint xy) -> (double) xy.y(),
-            (a, b) -> a + b,
+            Double::sum,
             TreeMap::new));
     return XySequence.create(
         combinedMap.keySet(),