"No valid plugin descriptors were found in META-INF/gradle-plugins"?

I’m trying to write a Gradle plugin. I get the following when I do ./gradlew build. What does this mean? I googled to no avail.

$ ./gradlew build
**> Task :jar**

:jar: No valid plugin descriptors were found in META-INF/gradle-plugins


5 actionable tasks: 3 executed, 2 from cache

$ ./gradlew -version

Gradle 4.10.2

Build time:   2018-09-19 18:10:15 UTC
Revision:     b4d8d5d170bb4ba516e88d7fe5647e2323d791dd

Kotlin DSL:   1.0-rc-6
Kotlin:       1.2.61
Groovy:       2.4.15
Ant:          Apache Ant(TM) version 1.9.11 compiled on March 23 2018
JVM:          1.8.0_181 (Oracle Corporation 25.181-b13)
OS:           Mac OS X 10.15.1 x86_64

See https://docs.gradle.org/current/userguide/custom_plugins.html