diff -r 7d15ebca4bb3 -r 17c09d1b3588 Admin/lib/Tools/build_setup
--- a/Admin/lib/Tools/build_setup	Tue Jul 06 12:36:47 2021 +0200
+++ b/Admin/lib/Tools/build_setup	Wed Jul 07 14:32:43 2021 +0200
@@ -57,7 +57,7 @@
   ARGS["${#ARGS[@]}"]="$(platform_path "$ISABELLE_HOME/src/Tools/Setup/isabelle/setup/$SRC")"
 done
 
-isabelle_jdk javac -d "$TARGET_DIR" "${ARGS[@]}" || \
+isabelle_jdk javac -d "$TARGET_DIR" -classpath "$(platform_path "$ISABELLE_CLASSPATH")" "${ARGS[@]}" || \
   fail "Failed to compile sources"
 
 isabelle_jdk jar -c -f "$(platform_path "$TARGET_DIR/isabelle_setup.jar")" \