diff --git a/README.md b/README.md
index 0f04fa88..fb5b4143 100644
--- a/README.md
+++ b/README.md
@@ -16,6 +16,10 @@ for instant access to methods and fields.
 ```
 > git clone https://github.com/java-decompiler/jd-gui.git
 > cd jd-gui
+# make sure jd-gui/src/proguard/resources/proguard.config.txt contains the correct path to rj.jar in -libraryjars
+# e.g. on Linux /usr/lib/jvm/java-1.8.0-openjdk-1.8.0.332.b09-1.fc35.x86_64/jre/lib/rt.jar
+# on Windows C:/Program Files/Java/jre1.8.0_121/lib/rt.jar
+# fixes errors like: can't find referenced class java.lang.String
 > ./gradlew build 
 ```
 generate :