diff --git a/sdks/java/core/src/main/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunner.java b/sdks/java/core/src/main/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunner.java
index d62bcc96e708..2415dabaedc3 100644
--- a/sdks/java/core/src/main/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunner.java
+++ b/sdks/java/core/src/main/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunner.java
@@ -438,8 +438,22 @@ private Collection<W> processElement(WindowedValue<InputT> value) throws Excepti
     for (BoundedWindow untypedWindow : value.getWindows()) {
       @SuppressWarnings("unchecked")
       W window = (W) untypedWindow;
+
+      ReduceFn<K, InputT, OutputT, W>.Context directContext =
+          contextFactory.base(window, StateStyle.DIRECT);
+      if (triggerRunner.isClosed(directContext.state())) {
+        // This window has already been closed.
+        droppedDueToClosedWindow.addValue(1L);
+        WindowTracing.debug(
+            "ReduceFnRunner.processElement: Dropping element at {} for key:{}; window:{} "
+            + "since window is no longer active at inputWatermark:{}; outputWatermark:{}",
+            value.getTimestamp(), key, window, timerInternals.currentInputWatermarkTime(),
+            timerInternals.currentOutputWatermarkTime());
+        continue;
+      }
+
       W active = activeWindows.representative(window);
-      Preconditions.checkState(active != null, "Window %s should have been added", window);
+      Preconditions.checkState(active != null, "Window %s has no representative", window);
       windows.add(active);
     }
 
@@ -450,24 +464,13 @@ private Collection<W> processElement(WindowedValue<InputT> value) throws Excepti
       triggerRunner.prefetchForValue(window, directContext.state());
     }
 
-    // Process the element for each (representative) window it belongs to.
+    // Process the element for each (representative, not closed) window it belongs to.
     for (W window : windows) {
       ReduceFn<K, InputT, OutputT, W>.ProcessValueContext directContext = contextFactory.forValue(
           window, value.getValue(), value.getTimestamp(), StateStyle.DIRECT);
       ReduceFn<K, InputT, OutputT, W>.ProcessValueContext renamedContext = contextFactory.forValue(
           window, value.getValue(), value.getTimestamp(), StateStyle.RENAMED);
 
-      // Check to see if the triggerRunner thinks the window is closed. If so, drop that window.
-      if (triggerRunner.isClosed(directContext.state())) {
-        droppedDueToClosedWindow.addValue(1L);
-        WindowTracing.debug(
-            "ReduceFnRunner.processElement: Dropping element at {} for key:{}; window:{} "
-            + "since window is no longer active at inputWatermark:{}; outputWatermark:{}",
-            value.getTimestamp(), key, window, timerInternals.currentInputWatermarkTime(),
-            timerInternals.currentOutputWatermarkTime());
-        continue;
-      }
-
       nonEmptyPanes.recordContent(renamedContext.state());
 
       // Make sure we've scheduled the end-of-window or garbage collection timer for this window.
diff --git a/sdks/java/core/src/test/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunnerTest.java b/sdks/java/core/src/test/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunnerTest.java
index 10b886b6e4d7..b58e360bd47d 100644
--- a/sdks/java/core/src/test/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunnerTest.java
+++ b/sdks/java/core/src/test/java/com/google/cloud/dataflow/sdk/util/ReduceFnRunnerTest.java
@@ -724,6 +724,44 @@ public void testMergingWithCloseBeforeGC() throws Exception {
         equalTo(PaneInfo.createPane(true, true, Timing.ON_TIME, 0, 0)));
   }
 
+  /**
+   * If a later event tries to reuse an earlier session window which has been closed, we
+   * should reject that element and not fail due to the window no longer having a representative.
+   */
+  @Test
+  public void testMergingWithReusedWindow() throws Exception {
+    ReduceFnTester<Integer, Iterable<Integer>, IntervalWindow> tester =
+        ReduceFnTester.nonCombining(Sessions.withGapDuration(Duration.millis(10)), mockTrigger,
+                                    AccumulationMode.DISCARDING_FIRED_PANES, Duration.millis(50),
+                                    ClosingBehavior.FIRE_IF_NON_EMPTY);
+
+    // One elements in one session window.
+    tester.injectElements(TimestampedValue.of(1, new Instant(1))); // in [1, 11), gc at 21.
+
+    // Close the trigger, but the gargbage collection timer is still pending.
+    when(mockTrigger.shouldFire(anyTriggerContext())).thenReturn(true);
+    triggerShouldFinish(mockTrigger);
+    tester.advanceInputWatermark(new Instant(15));
+
+    // Another element in the same session window.
+    // Should be discarded with 'window closed'.
+    tester.injectElements(TimestampedValue.of(1, new Instant(1))); // in [1, 11), gc at 21.
+
+    // Now the garbage collection timer will fire, finding the trigger already closed.
+    tester.advanceInputWatermark(new Instant(100));
+
+    List<WindowedValue<Iterable<Integer>>> output = tester.extractOutput();
+    assertThat(output.size(), equalTo(1));
+    assertThat(output.get(0),
+               isSingleWindowedValue(containsInAnyOrder(1),
+                                     1, // timestamp
+                                     1, // window start
+                                     11)); // window end
+    assertThat(
+        output.get(0).getPane(),
+        equalTo(PaneInfo.createPane(true, true, Timing.ON_TIME, 0, 0)));
+  }
+
   /**
    * Tests that when data is assigned to multiple windows but some of those windows have
    * had their triggers finish, then the data is dropped and counted accurately.