diff --git a/player-piano/README.md b/player-piano/README.md
index db4a48f..b2829a3 100644
--- a/player-piano/README.md
+++ b/player-piano/README.md
@@ -42,3 +42,46 @@ The player piano takes the basic commands created by the ingrid code (e.g., crea
 Once the macro is loaded, there will be a `Tools > Macros > Player Piano` menu item. Use this to launch the macro.
 
 The script is written in Groovy, and can be activated in MagicDraw through the Macro Engine. See [MagicDraw Online Documentation](https://docs.nomagic.com/display/MD190/Adding+a+Macro+and+editing+Macro+information) for instructions on how to do this.
+
+## Known Issues
+
+### Player Piano Macro Does Not Launch in Cameo 2021xR2
+
+The Player Piano macro in Cameo Systems Modeler 2021xR2 produces an error:
+
+```
+MagicDraw cannot execute the Groovy macro, please make sure that the C:\Users\test_user\rapid-modeling-tools-master\player-piano\player-piano-script.groovy is correct. startup failed: C:\Users\test_user\rapid-modeling-tools-master\player-piano\player-piano-script.groovy: 142: unable to resolve class JsonSlurper @ line 142, column 18. jsonSlurper = new JsonSlurper(); ^ 1 error org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed: C:\Users\test_user\rapid-modeling-tools-master\player-piano\player-piano-script.groovy: 142: unable to resolve class JsonSlurper @ line 142, column 18. jsonSlurper = new JsonSlurper(); ^ 1 error
+```
+
+The root issue is that Cameo 2021xR2 ships with the core
+Groovy 3.0.7 JAR module that since
+[version 2](http://groovy-lang.org/releasenotes/groovy-2.0.html#Groovy20releasenotes-AmoremodularGroovy)
+does not include JSON utilities. (Both Cameo versions 2021xR1 and 190sp4 ship with the Groovy 2.4.15 `all` JAR that contains everything.)
+
+Therefore, we need to update the plugin that executes Groovy scripts to explicitly import the JSON module.
+
+1. Go to the root of your Cameo 2021xR2 installation.
+  You should see a `plugins` folder.
+2. Download the
+   [groovy-json-3.0.7.jar](https://repo1.maven.org/maven2/org/codehaus/groovy/groovy-json/3.0.7/groovy-json-3.0.7.jar)
+   to
+   ```
+   plugins\com.nomagic.magicdraw.automaton\lib\engines\groovy-3.0.7\lib
+   ```
+3. Open the plugin configuration in a text editor:
+   ```
+   plugins\com.nomagic.magicdraw.automaton\plugin.xml
+   ```
+   You should see a `<library>` XML element importing
+   the core Groovy 3.0.7 JAR:
+   ```
+   <library name="lib/engines/groovy-3.0.7/lib/groovy-3.0.7.jar"/>
+   ```
+4. Add this line to `plugin.xml`
+   just below the core Groovy import:
+   ```
+   <library name="lib/engines/groovy-3.0.7/lib/groovy-json-3.0.7.jar"/>
+   ```
+   This points the plugin to the module you downloaded.
+5. Run Cameo 2021xR2 and execute the player piano macro.
+   You should see a "Select Package" window appear without errors.
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