diff --git a/sample-jar/Makefile b/sample-jar/Makefile
new file mode 100644
index 0000000..be11926
--- /dev/null
+++ b/sample-jar/Makefile
@@ -0,0 +1,8 @@
+sample.jar: classes/a/A.class
+	jar -c -f $@ $^
+
+classes/a/A.class: src/a/A.java
+	javac -d classes src/a/A.java
+
+clean:
+	rm -rf classes
diff --git a/sample-jar/classes/a/A.class b/sample-jar/classes/a/A.class
new file mode 100644
index 0000000..ea6785d
Binary files /dev/null and b/sample-jar/classes/a/A.class differ
diff --git a/sample-jar/sample.jar b/sample-jar/sample.jar
new file mode 100644
index 0000000..03c5af9
Binary files /dev/null and b/sample-jar/sample.jar differ
diff --git a/sample-jar/src/a/A.java b/sample-jar/src/a/A.java
new file mode 100644
index 0000000..e367656
--- /dev/null
+++ b/sample-jar/src/a/A.java
@@ -0,0 +1,3 @@
+package a;
+
+public class A {}
diff --git a/src/main/java/no/ion/jhms/modularizer/ModuleDescription.java b/src/main/java/no/ion/jhms/modularizer/ModuleDescription.java
index 9ab52ff..9b72e9d 100644
--- a/src/main/java/no/ion/jhms/modularizer/ModuleDescription.java
+++ b/src/main/java/no/ion/jhms/modularizer/ModuleDescription.java
@@ -26,6 +26,11 @@ public Options setJarPath(Path path) {
         public PrintStream out() { return out; }
     }
 
+    /**
+     * Writes a description of the modular JAR given by {@link Options#jarPath()}.
+     *
+     * @throws ErrorException if jar file is not a module
+     */
     public static void describeModule(PrintStream out, Options options) {
         new ModuleDescription(out, options).describe();
     }
diff --git a/src/test/java/no/ion/jhms/modularizer/ModuleUpdaterTest.java b/src/test/java/no/ion/jhms/modularizer/ModuleUpdaterTest.java
index d28df11..cf0c5fc 100644
--- a/src/test/java/no/ion/jhms/modularizer/ModuleUpdaterTest.java
+++ b/src/test/java/no/ion/jhms/modularizer/ModuleUpdaterTest.java
@@ -1,17 +1,59 @@
 package no.ion.jhms.modularizer;
 
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
-import java.lang.module.ModuleDescriptor;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.PrintStream;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
 import java.nio.file.Path;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
 
 class ModuleUpdaterTest {
+    private final ByteArrayOutputStream baos = new ByteArrayOutputStream();
+    private final PrintStream printStream = new PrintStream(baos);
+    private Path path;
+
+    @TempDir
+    Path tmpDirectory;
+
+    @BeforeEach
+    void setUp() {
+        path = tmpDirectory.resolve("sample.jar");
+    }
+
     @Test
-    public void addModuleInfoClass() {
-        ModuleInfoClassReader reader = ModuleInfoClassReader.open(Path.of("src/test/resources/min-descriptor/module-info.class"));
-        ModuleInfoClass moduleInfoClass = reader.parse();
+    public void verifyUpdate() throws IOException {
+        Files.copy(Path.of("sample-jar/sample.jar"), path);
+
+        var describeOptions = new ModuleDescription.Options().setJarPath(path);
+        try {
+            ModuleDescription.describeModule(printStream, describeOptions);
+            fail();
+        } catch (ErrorException e) {
+            assertTrue(e.getMessage().startsWith("error: no module declaration found:"), e::getMessage);
+            assertTrue(e.getMessage().endsWith("/sample.jar"));
+        }
+        assertEquals("", output());
+
+        var updateOptions = new ModuleUpdater.Options()
+                .setJarPath(path)
+                .setModuleName("module.name");
+        ModuleUpdater.update(printStream, updateOptions);
+        assertEquals("", output());
+    }
+
+    private String getDeclaration() {
+
+    }
+
+    private String output() {
+        return baos.toString(StandardCharsets.UTF_8);
     }
 }