Why admin client throw the error "couldn't determine local port"?

I've configure admin client for each microservice as the following.
First from the pom file:

 <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>     
        <dependency>
            <groupId>de.codecentric</groupId>
            <artifactId>spring-boot-admin-starter-client</artifactId>          
        </dependency>

Then from the application.properties file:

# Application
spring.application.name=My Sample Service

# Spring boot admin server 
spring.boot.admin.client.url=http://localhost:9091
spring.boot.admin.client.enabled=true
spring.boot.admin.client.auto-registration=true
spring.boot.admin.client.username=admin
spring.boot.admin.client.password=password

spring.security.user.name=admin
spring.security.user.password=password
spring.boot.admin.client.instance.metadata.user.name=${spring.security.user.name}
spring.boot.admin.client.instance.metadata.user.password=${spring.security.user.password}


# Actuator endpoints properties
management.endpoints.web.exposure.include=*
management.endpoint.health.show-details=always

# To monitor log file on admin server
logging.file.name=my_ssample_service.log
logging.logback.rollingpolicy.max-history=5
logging.logback.rollingpolicy.max-file-size=10MB

When running mvn install (the build is successful) I get the following error:

java.lang.IllegalStateException: couldn't determine local port. Please set spring.boot.admin.client.instance.service-base-url.
	at de.codecentric.boot.admin.client.registration.DefaultApplicationFactory.getLocalServerPort(DefaultApplicationFactory.java:192) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at de.codecentric.boot.admin.client.registration.DefaultApplicationFactory.getServiceBaseUrl(DefaultApplicationFactory.java:104) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at de.codecentric.boot.admin.client.registration.ServletApplicationFactory.getServiceUrl(ServletApplicationFactory.java:63) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at de.codecentric.boot.admin.client.registration.ServletApplicationFactory.getManagementBaseUrl(ServletApplicationFactory.java:76) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at de.codecentric.boot.admin.client.registration.DefaultApplicationFactory.getHealthUrl(DefaultApplicationFactory.java:154) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at de.codecentric.boot.admin.client.registration.DefaultApplicationFactory.createApplication(DefaultApplicationFactory.java:80) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at de.codecentric.boot.admin.client.registration.DefaultApplicationRegistrator.register(DefaultApplicationRegistrator.java:56) ~[spring-boot-admin-client-2.7.4.jar:2.7.4]
	at org.springframework.scheduling.support.DelegatingErrorHandlingRunnable.run(DelegatingErrorHandlingRunnable.java:54) ~[spring-context-5.3.22.jar:5.3.22]

Each client application has port configured in application.yml

server:
  port : 9020

I must say that the admin server is able to show all the clients' details correctly which means it has access to all clients actuator urls. But the error is annoying.

Adding the following spring boot property for each microservice client solve
the build error problem:

spring.boot.admin.client.instance.service-base-url=http://localhost:9020

Where 9020 is the port of the admin client (not server). Also it appears that this
property is being used by spring test when building with mvn install.

Now all builds works with no errors. hope it helps someone who has similar issue.