SonarScanner values arent getting replaced

Hi

Am trying to do PR analysis using Git-Jenkins-Sonar integration

Everytime i pass the environment variables to the script , it is replacing null

	 sh '''/var/lib/jenkins/tools/hudson.plugins.sonar.SonarRunnerInstallation/SonarQube_Scanner_2.9/bin/sonar-scanner -Duser.home=/data/jenkins-misc/sonar-cache -Dsonar.host.url=https://sonarqube.dhl.com -Dsonar.projectBaseDir=\$WORKSPACE -Dsonar.projectKey=RPAPlatormStandards -Dsonar.github.oauth=${githubToken} -Dsonar.github.pullRequest=${prNo}'''

Instead of replacing PrNo with value it is replacing null there

Any idea what could be wrong ?