jmod: "Error: module-info.class not found" despite it existing in the jar file

Question

Attempting to create a .mod file from a jar file.

 $JAVA_HOME/bin/jmod create --class-path ./bouncycastle/bcprov-jdk18on-171.jar bouncycastle.mod

results in:

Error: module-info.class not found
java.io.IOException: module-info.class not found
    at jdk.jlink/jdk.tools.jmod.JmodTask$JmodFileWriter.writeModuleInfo(JmodTask.java:538)
    at jdk.jlink/jdk.tools.jmod.JmodTask$JmodFileWriter.write(JmodTask.java:476)
    at jdk.jlink/jdk.tools.jmod.JmodTask.create(JmodTask.java:433)
    at jdk.jlink/jdk.tools.jmod.JmodTask.run(JmodTask.java:208)
    at jdk.jlink/jdk.tools.jmod.Main.main(Main.java:34)

that classfile does exist though, and it is valid

jar -tvf bouncycastle/bcprov-jdk18on-171.jar | grep module

  7672 Thu Mar 31 10:06:58 PDT 2022 META-INF/versions/9/module-info.class

What am I doing wrong here?