diff --git a/metamorph/build.gradle b/metamorph/build.gradle
index ed537f1a5..5fd1c5121 100644
--- a/metamorph/build.gradle
+++ b/metamorph/build.gradle
@@ -25,6 +25,9 @@ dependencies {
   implementation project(':metafacture-mangling')
   implementation project(':metafacture-javaintegration')
   implementation 'org.slf4j:slf4j-api:1.7.21'
+  testImplementation project(':metafacture-io')
+  testImplementation project(':metafacture-json')
+  testImplementation project(':metafacture-strings')
   testImplementation 'junit:junit:4.12'
   testImplementation 'org.mockito:mockito-core:2.5.5'
   testRuntimeOnly 'org.slf4j:slf4j-simple:1.7.21'
@@ -45,4 +48,6 @@ test {
     showStandardStreams = true
     exceptionFormat = 'full'
   }
+
+  systemProperties System.properties
 }
diff --git a/metamorph/src/test/java/org/metafacture/metamorph/Issue366Test.java b/metamorph/src/test/java/org/metafacture/metamorph/Issue366Test.java
new file mode 100644
index 000000000..bed96a453
--- /dev/null
+++ b/metamorph/src/test/java/org/metafacture/metamorph/Issue366Test.java
@@ -0,0 +1,96 @@
+package org.metafacture.metamorph;
+
+import org.metafacture.io.LineReader;
+import org.metafacture.io.ObjectJavaIoWriter;
+import org.metafacture.json.JsonDecoder;
+import org.metafacture.json.JsonEncoder;
+import org.metafacture.strings.StringReader;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.io.StringWriter;
+import java.io.Writer;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.atomic.LongAdder;
+import java.util.stream.IntStream;
+
+public final class Issue366Test {
+
+    private static final String INPUT = "{\"10012\":{\"a\":\"V\"}}";
+
+    private static final String[] OUTPUT = new String[]{
+        "{\"r\":{\"k\":\"K\",\"v\":\"V\"}}\n",
+        "{\"r\":{\"v\":\"V\",\"k\":\"K\"}}\n",
+        "{\"r\":{\"k\":\"K\",\"v\":\"V\"}}\n",
+        "{\"r\":{\"k\":\"K\",\"v\":\"V\"}}\n"
+    };
+
+    private static final String MORPH = "" +
+        "<metamorph xmlns=\"http://www.culturegraph.org/metamorph\" version=\"1\">\n" +
+        "  <rules>\n" +
+        "    <entity name=\"r\" flushWith=\"100??\">\n" +
+        "      <choose flushWith=\"100%3$s%1$s\">\n" +
+        "        <data name=\"k\" source=\"100%2$s.a\">\n" +
+        "          <constant value=\"K\"/>\n" +
+        "        </data>\n" +
+        "      </choose>\n" +
+        "      <data name=\"v\" source=\"100%3$s.a\" />\n" +
+        "    </entity>\n" +
+        "  </rules>\n" +
+        "</metamorph>";
+
+    private static final int OPTION = Integer.valueOf(System.getProperty("issue366.option", "0"));
+
+    private static final int ITER = Integer.valueOf(System.getProperty("issue366.iter", "10"));
+
+    private final Map<String, LongAdder> outcome = new HashMap<>();
+
+    private final Object[] args = new String[]{".a", "?2", "??"};
+    // Either one of these replacements avoids the issue:
+    private final Object[] opts = new String[]{"",   "??", "?2"};
+
+    @Test
+    public void testTransform() {
+        if (OPTION > args.length) {
+            throw new RuntimeException("Invalid option: " + OPTION);
+        }
+        else if (OPTION > 0) {
+            args[OPTION - 1] = opts[OPTION - 1];
+        }
+
+        final String morph = String.format(MORPH, args);
+        System.out.println(morph);
+
+        IntStream.range(0, ITER).forEach(i -> transform(morph));
+        outcome.forEach((k, v) -> System.err.print(v.sum() + "/" + ITER + ": " + k));
+
+        Assert.assertEquals(1, outcome.size());
+        Assert.assertEquals(OUTPUT[OPTION], outcome.keySet().iterator().next());
+    }
+
+    private void transform(final String morph) {
+        final StringReader reader = new StringReader();
+        final Writer writer = new StringWriter();
+
+        reader
+            .setReceiver(new LineReader())
+            .setReceiver(new JsonDecoder())
+            .setReceiver(new Metamorph(new java.io.StringReader(morph)))
+            .setReceiver(new JsonEncoder())
+            // ObjectJavaIoWriter in org.metafacture.io has been deprecated
+            .setReceiver(new ObjectJavaIoWriter<>(writer));
+
+        try {
+            reader.process(INPUT);
+            reader.closeStream();
+
+            outcome.computeIfAbsent(writer.toString(), k -> new LongAdder()).increment();
+        }
+        catch (Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+}