diff --git a/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java b/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java
new file mode 100644
index 0000000..ddc03a8
--- /dev/null
+++ b/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java
@@ -0,0 +1,52 @@
+// Copyright (c) 2023, the R8 project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+package com.android.tools.r8.mappingcompose;
+
+import static org.junit.Assert.assertThrows;
+
+import com.android.tools.r8.TestBase;
+import com.android.tools.r8.TestParameters;
+import com.android.tools.r8.TestParametersCollection;
+import com.android.tools.r8.errors.Unreachable;
+import com.android.tools.r8.naming.ClassNameMapper;
+import com.android.tools.r8.naming.MappingComposer;
+import com.android.tools.r8.utils.StringUtils;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+import org.junit.runners.Parameterized.Parameter;
+import org.junit.runners.Parameterized.Parameters;
+
+@RunWith(Parameterized.class)
+public class ComposeUnknownJsonTest extends TestBase {
+
+  @Parameter() public TestParameters parameters;
+
+  @Parameters(name = "{0}")
+  public static TestParametersCollection data() {
+    return getTestParameters().withNoneRuntime().build();
+  }
+
+  private static final String mappingFoo =
+      StringUtils.unixLines(
+          "# { id: 'com.android.tools.r8.mapping', version: '2.2' }",
+          "# { id: 'some.unknown.identifier', settings: 'message1' }",
+          "com.foo -> a:",
+          "# { id: 'some.other.unknown.identifier', foo: 'message2' }");
+  private static final String mappingBar =
+      StringUtils.unixLines(
+          "# { id: 'com.android.tools.r8.mapping', version: '2.2' }",
+          "# { id: 'some.third.unknown.identifier', bar: 'message3' }",
+          "a -> b:",
+          "# { id: 'some.fourth.unknown.identifier', baz: 'message4' }");
+
+  @Test
+  public void testCompose() throws Exception {
+    ClassNameMapper mappingForFoo = ClassNameMapper.mapperFromStringWithPreamble(mappingFoo);
+    ClassNameMapper mappingForBar = ClassNameMapper.mapperFromStringWithPreamble(mappingBar);
+    // TODO(b/241763080): We should not throw when composing mapping information.
+    assertThrows(Unreachable.class, () -> MappingComposer.compose(mappingForFoo, mappingForBar));
+  }
+}