diff --git a/src/engine/AddCombinedRowToTable.h b/src/engine/AddCombinedRowToTable.h
index c43e298a88..45f51fb527 100644
--- a/src/engine/AddCombinedRowToTable.h
+++ b/src/engine/AddCombinedRowToTable.h
@@ -155,8 +155,13 @@ class AddCombinedRowToIdTable {
     if (nextIndex_ != 0) {
       AD_CORRECTNESS_CHECK(inputLeftAndRight_.has_value());
       flush();
-    } else {
-      // Clear vocab when no rows were written.
+    } else if (resultTable_.empty()) {
+      // Clear local vocab when no rows were written.
+      //
+      // TODO<joka921, robinTF> This is a conservative approach. We could
+      // optimize this case (clear the local vocab more often, but still
+      // correctly) by considering the situation after all the relevant inputs
+      // have been processed.
       mergedVocab_ = LocalVocab{};
     }
   }
diff --git a/test/AddCombinedRowToTableTest.cpp b/test/AddCombinedRowToTableTest.cpp
index 2c409a830b..4bf72adcf9 100644
--- a/test/AddCombinedRowToTableTest.cpp
+++ b/test/AddCombinedRowToTableTest.cpp
@@ -323,3 +323,39 @@ TEST(AddCombinedRowToTable, verifyLocalVocabIsRetainedWhenNotMoving) {
   EXPECT_TRUE(vocabContainsString(localVocab, "b"));
   EXPECT_THAT(localVocab.getAllWordsForTesting(), ::testing::SizeIs(2));
 }
+
+// _____________________________________________________________________________
+TEST(AddCombinedRowToTable, localVocabIsOnlyClearedWhenLegal) {
+  auto outputTable = makeIdTableFromVector({});
+  outputTable.setNumColumns(3);
+  ad_utility::AddCombinedRowToIdTable adder{
+      1, std::move(outputTable),
+      std::make_shared<ad_utility::CancellationHandle<>>(), 1};
+
+  IdTableWithVocab input1{makeIdTableFromVector({{0, 1}}),
+                          createVocabWithSingleString("a")};
+  IdTableWithVocab input2{makeIdTableFromVector({{0, 2}}),
+                          createVocabWithSingleString("b")};
+
+  adder.setInput(input1, input2);
+  adder.addRow(0, 0);
+  IdTableWithVocab input3{makeIdTableFromVector({{3, 1}}),
+                          createVocabWithSingleString("c")};
+  IdTableWithVocab input4{makeIdTableFromVector({{3, 2}}),
+                          createVocabWithSingleString("d")};
+  // NOTE: This seemingly redundant call to `setInput` is important, as it tests
+  // a previous bug: Each call to `setInput` implicitly also calls `flush` and
+  // also possibly clears the local vocab if it is not used anymore. In this
+  // case however we may not clear the local vocab, as the result of the
+  // previous calls to `addRow` has not yet been extracted.
+  adder.setInput(input1, input2);
+  adder.setInput(input3, input4);
+  adder.addRow(0, 0);
+  auto localVocab = adder.localVocab().clone();
+
+  EXPECT_TRUE(vocabContainsString(localVocab, "a"));
+  EXPECT_TRUE(vocabContainsString(localVocab, "b"));
+  EXPECT_TRUE(vocabContainsString(localVocab, "c"));
+  EXPECT_TRUE(vocabContainsString(localVocab, "d"));
+  EXPECT_THAT(localVocab.getAllWordsForTesting(), ::testing::SizeIs(4));
+}