Getting an error when trying to run the `simple` project

sbt
[info] welcome to sbt 1.6.2 (Amazon.com Inc. Java 18.0.1)
[info] loading global plugins from /Users/myuser/.sbt/1.0/plugins
java.lang.Exception: The system property 'plugin.version' is not defined.
at $09e901f3bb9bfda1e449$.$sbtdef(/Users/myuser/dev/sandbox/sbt-openapi-generator/src/sbt-test/sbt-openapi-generator/simple/project/plugin.sbt:7)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
at java.base/java.lang.reflect.Method.invoke(Method.java:577)
at sbt.compiler.Eval$.getValue(Eval.scala:601)
at sbt.compiler.Eval.$anonfun$eval$1(Eval.scala:133)
at sbt.internal.EvaluateConfigurations$.$anonfun$evaluateDslEntry$1(EvaluateConfigurations.scala:249)
at sbt.internal.EvaluateConfigurations$.$anonfun$evaluateSbtFile$6(EvaluateConfigurations.scala:172)
at scala.collection.immutable.List.map(List.scala:297)
at sbt.internal.EvaluateConfigurations$.$anonfun$evaluateSbtFile$4(EvaluateConfigurations.scala:172)
at sbt.internal.Load$.loadSettingsFile$1(Load.scala:1121)
at sbt.internal.Load$.$anonfun$discoverProjects$2(Load.scala:1130)
at scala.collection.MapLike.getOrElse(MapLike.scala:131)
at scala.collection.MapLike.getOrElse$(MapLike.scala:129)
at scala.collection.AbstractMap.getOrElse(Map.scala:65)

Reproduce the failure by running sbt in src/sbt-test/sbt-openapi-generator/simple. Start with project/plugin.sbt at line 7, where the missing system property is reported; done means the simple project loads without this exception.