Why does IntelliJ say "Variables debug info not available"

Question

I have a simple java program that is managed by maven.

I run it on the commandline like this:

mvnDebug exec:java -Dexec.mainClass="Main" 

and then attach IntelliJ to the java program.

My pom.xml looks like:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.2</version>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
                <compilerArgs>
                    <arg>-verbose</arg>
                    <arg>-g</arg>
                </compilerArgs>
            </configuration>
        </plugin>

When the control goes into rt.jar, IntelliJ says: "Variables debug info not available"

Here is the maven version:

$ mvn --version
Apache Maven 3.5.0 (ff8f5e7444045639af65f6095c62210b5713f426; 2017-04-03T12:39:06-07:00)
Maven home: /usr/local/Cellar/maven/3.5.0/libexec
Java version: 1.8.0_144, vendor: Oracle Corporation
Java home: /Library/Java/JavaVirtualMachines/jdk1.8.0_144.jdk/Contents/Home/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "mac os x", version: "10.12.6", arch: "x86_64", family: "mac"