diff --git a/components/core/src/clp/ffi/ir_stream/Serializer.cpp b/components/core/src/clp/ffi/ir_stream/Serializer.cpp
index 379330373..09199e32d 100644
--- a/components/core/src/clp/ffi/ir_stream/Serializer.cpp
+++ b/components/core/src/clp/ffi/ir_stream/Serializer.cpp
@@ -550,6 +550,9 @@ auto Serializer<encoded_variable_t>::create(
             cVariableEncodingMethodsVersion
     );
     if (optional_user_defined_metadata.has_value()) {
+        if (false == optional_user_defined_metadata.value().is_object()) {
+            return std::errc::protocol_not_supported;
+        }
         metadata.emplace(
                 string{cProtocol::Metadata::UserDefinedMetadataKey},
                 std::move(optional_user_defined_metadata.value())
diff --git a/components/core/src/clp/ffi/ir_stream/Serializer.hpp b/components/core/src/clp/ffi/ir_stream/Serializer.hpp
index 3906c4d6e..8fc40a77c 100644
--- a/components/core/src/clp/ffi/ir_stream/Serializer.hpp
+++ b/components/core/src/clp/ffi/ir_stream/Serializer.hpp
@@ -41,9 +41,11 @@ class Serializer {
     // Factory functions
     /**
      * Creates an IR serializer and serializes the stream's preamble.
-     * @param optional_user_defined_metadata Stream-level user-defined metadata.
+     * @param optional_user_defined_metadata Stream-level user-defined metadata, given as a JSON
+     * object.
      * @return A result containing the serializer or an error code indicating the failure:
-     * - std::errc::protocol_error on failure to serialize the preamble.
+     * - std::errc::protocol_error if failed to serialize the preamble.
+     * - std::errc::protocol_not_supported if the given user-defined metadata is not a JSON object.
      */
     [[nodiscard]] static auto create(
             std::optional<nlohmann::json> optional_user_defined_metadata = std::nullopt
diff --git a/components/core/tests/test-ir_encoding_methods.cpp b/components/core/tests/test-ir_encoding_methods.cpp
index 97786c92f..361f96c67 100644
--- a/components/core/tests/test-ir_encoding_methods.cpp
+++ b/components/core/tests/test-ir_encoding_methods.cpp
@@ -1494,3 +1494,23 @@ TEMPLATE_TEST_CASE(
     };
     REQUIRE(assert_invalid_serialization(array_with_invalid_submap));
 }
+
+// NOLINTNEXTLINE(readability-function-cognitive-complexity)
+TEMPLATE_TEST_CASE(
+        "ffi_ir_stream_Serializer_serialize_invalid_user_defined_metadata",
+        "[clp][ffi][ir_stream][Serializer]",
+        four_byte_encoded_variable_t,
+        eight_byte_encoded_variable_t
+) {
+    auto invalid_user_defined_metadata = GENERATE(
+            nlohmann::json(std::string{"str"}),
+            nlohmann::json(int{0}),
+            nlohmann::json(double{0.0}),
+            nlohmann::json(true),
+            nlohmann::json(nullptr),
+            nlohmann::json(vector<int>{0, 1, 2})
+    );
+    auto const serializer_result{Serializer<TestType>::create(invalid_user_defined_metadata)};
+    REQUIRE(serializer_result.has_error());
+    REQUIRE((std::errc::protocol_not_supported == serializer_result.error()));
+}