diff --git a/examples/pom.xml b/examples/pom.xml
index 22183670c2d48..c15f73f688ca8 100644
--- a/examples/pom.xml
+++ b/examples/pom.xml
@@ -22,7 +22,7 @@
   <parent>
     <groupId>com.google.cloud.dataflow</groupId>
     <artifactId>google-cloud-dataflow-java-sdk-parent</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
   </parent>
 
   <groupId>com.google.cloud.dataflow</groupId>
diff --git a/pom.xml b/pom.xml
index 7e5e078339e71..6fb0b32a573e2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -35,7 +35,7 @@
   <url>http://cloud.google.com/dataflow</url>
   <inceptionYear>2013</inceptionYear>
 
-  <version>1.5.0-SNAPSHOT</version>
+  <version>1.6.0-SNAPSHOT</version>
 
   <licenses>
     <license>
diff --git a/sdk/pom.xml b/sdk/pom.xml
index f782b78532474..d7e10a53a8018 100644
--- a/sdk/pom.xml
+++ b/sdk/pom.xml
@@ -22,7 +22,7 @@
   <parent>
     <groupId>com.google.cloud.dataflow</groupId>
     <artifactId>google-cloud-dataflow-java-sdk-parent</artifactId>
-    <version>1.5.0-SNAPSHOT</version>
+    <version>1.6.0-SNAPSHOT</version>
   </parent>
 
   <groupId>com.google.cloud.dataflow</groupId>
diff --git a/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/MergingActiveWindowSet.java b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/MergingActiveWindowSet.java
index 5af4ea5676836..96629b1601efb 100644
--- a/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/MergingActiveWindowSet.java
+++ b/sdk/src/main/java/com/google/cloud/dataflow/sdk/util/MergingActiveWindowSet.java
@@ -191,8 +191,12 @@ public void addActive(W window) {
 
   @Override
   public void remove(W window) {
-    Preconditions.checkState(isActive(window), "Window %s is not active", window);
-    for (W stateAddressWindow : activeWindowToStateAddressWindows.get(window)) {
+    Set<W> stateAddressWindows = activeWindowToStateAddressWindows.get(window);
+    if (stateAddressWindows == null) {
+      // Window is no longer active.
+      return;
+    }
+    for (W stateAddressWindow : stateAddressWindows) {
       windowToActiveWindow.remove(stateAddressWindow);
     }
     activeWindowToStateAddressWindows.remove(window);