diff --git a/README.md b/README.md
index 085ed80..f921bb5 100644
--- a/README.md
+++ b/README.md
@@ -53,7 +53,7 @@ Please also check the documentation of [Language Support for Java by Red Hat](ht
 - `classPaths` - The classpaths for launching the JVM. If not specified, the debugger will automatically resolve from current project.
 - `encoding` - The `file.encoding` setting for the JVM. If not specified, 'UTF-8' will be used. Possible values can be found in http://docs.oracle.com/javase/8/docs/technotes/guides/intl/encoding.doc.html.
 - `vmArgs` - The extra options and system properties for the JVM (e.g. -Xms\<size\> -Xmx\<size\> -D\<name\>=\<value\>).
-- `projectName` - The preferred project in which the debugger searches for classes. There could be duplicated class names in different projects. This setting also works when the debugger looks for the specified main class when launching a program.
+- `projectName` - The preferred project in which the debugger searches for classes. There could be duplicated class names in different projects. This setting also works when the debugger looks for the specified main class when launching a program. It is required for expression evaluation.
 - `cwd` - The working directory of the program.
 - `env` - The extra environment variables for the program.
 - `stopOnEntry` - Automatically pause the program after launching.
diff --git a/package.json b/package.json
index e501cff..7771011 100644
--- a/package.json
+++ b/package.json
@@ -56,7 +56,7 @@
             "properties": {
               "projectName": {
                 "type": "string",
-                "description": "The preferred project in which the debugger searches for classes. There could be duplicated class names in different projects. This setting also works when the debugger looks for the specified main class when launching a program.",
+                "description": "The preferred project in which the debugger searches for classes. There could be duplicated class names in different projects. This setting also works when the debugger looks for the specified main class when launching a program. It is required for expression evaluation.",
                 "default": ""
               },
               "mainClass": {