diff --git a/sdks/java/extensions/euphoria/euphoria-core/src/main/java/cz/seznam/euphoria/core/executor/greduce/GroupReducer.java b/sdks/java/extensions/euphoria/euphoria-core/src/main/java/cz/seznam/euphoria/core/executor/greduce/GroupReducer.java
index 5495ff92cb83b..5caf310ecc67d 100644
--- a/sdks/java/extensions/euphoria/euphoria-core/src/main/java/cz/seznam/euphoria/core/executor/greduce/GroupReducer.java
+++ b/sdks/java/extensions/euphoria/euphoria-core/src/main/java/cz/seznam/euphoria/core/executor/greduce/GroupReducer.java
@@ -192,7 +192,11 @@ private Pair<WID, Trigger.TriggerResult> mergeWindows(WID newWindow) {
       //  c) the target window's trigger #onClear won't be called
       sources.remove(target);
 
-      // XXX only if sources non empty!
+      // ~ do not bother with the rest of thi for loop if we have
+      // no source windows to merge
+      if (sources.isEmpty()) {
+        continue;
+      }
 
       // ~ make sure to create the target state if necessary
       State targetState = getStateForUpdate(target);