diff --git a/velox/functions/lib/MapConcat.cpp b/velox/functions/lib/MapConcat.cpp
index 81c415f693c3..aac38fbc9315 100644
--- a/velox/functions/lib/MapConcat.cpp
+++ b/velox/functions/lib/MapConcat.cpp
@@ -157,7 +157,7 @@ class MapConcatFunction : public exec::VectorFunction {
   static std::vector<std::shared_ptr<exec::FunctionSignature>> signatures() {
     // map(K,V), map(K,V), ... -> map(K,V)
     return {exec::FunctionSignatureBuilder()
-                .knownTypeVariable("K")
+                .typeVariable("K")
                 .typeVariable("V")
                 .returnType("map(K,V)")
                 .argumentType("map(K,V)")
diff --git a/velox/functions/lib/tests/MapConcatTest.cpp b/velox/functions/lib/tests/MapConcatTest.cpp
index 9a46d90b5c94..c269391e1252 100644
--- a/velox/functions/lib/tests/MapConcatTest.cpp
+++ b/velox/functions/lib/tests/MapConcatTest.cpp
@@ -283,3 +283,14 @@ TEST_F(MapConcatTest, nullEntry) {
   ASSERT_EQ(result->size(), 1);
   EXPECT_EQ(result->sizeAt(0), 0);
 }
+
+TEST_F(MapConcatTest, unknownType) {
+  // MAP_CONCAT(MAP[], MAP[])
+  auto emptyMapVector =
+      VectorTestBase::makeMapVector<UnknownValue, UnknownValue>({{}});
+  auto expectedMap =
+      VectorTestBase::makeMapVector<UnknownValue, UnknownValue>({{}});
+  auto result = evaluate(
+      "map_concat(c0, c1)", makeRowVector({emptyMapVector, emptyMapVector}));
+  facebook::velox::test::assertEqualVectors(expectedMap, result);
+}