diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index ab1dc8d..fe4fb2b 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -8,6 +8,7 @@ variables:
   CONFIGURATION: 'Release'
   BUILD_PRERELEASE: true
   DOTNET_CORE_VERSION: 8.x
+  PR_NUMBER: $[coalesce(variables['System.PullRequest.PullRequestNumber'], '0')]
 
 jobs:
   - job: pack
@@ -26,7 +27,7 @@ jobs:
           dotnet tool restore
         displayName: Restore .NET tools
       - pwsh: |
-          dotnet cake --target=Pack --configuration="$(CONFIGURATION)" --packageVersion="$(PACKAGE_VERSION)" --prerelease="$(BUILD_PRERELEASE)" --pullrequest="$env:SYSTEM_PULLREQUEST_PULLREQUESTNUMBER"
+          dotnet cake --target=Pack --configuration="$(CONFIGURATION)" --packageVersion="$(PACKAGE_VERSION)" --prerelease="$(BUILD_PRERELEASE)" --pullrequest="$(PR_NUMBER)"
         displayName: Pack
       - task: PublishBuildArtifacts@1
         displayName: Publish NuGet packages
@@ -50,7 +51,7 @@ jobs:
           dotnet tool restore
         displayName: Restore .NET tools
       - pwsh: |
-          dotnet cake --target=Build --configuration="$(CONFIGURATION)" --packageVersion="$(PACKAGE_VERSION)" --prerelease="$(BUILD_PRERELEASE)" --pullrequest="$env:SYSTEM_PULLREQUEST_PULLREQUESTNUMBER"
+          dotnet cake --target=Build --configuration="$(CONFIGURATION)" --packageVersion="$(PACKAGE_VERSION)" --prerelease="$(BUILD_PRERELEASE)" --pullrequest="$(PR_NUMBER)"
         displayName: Build
 
   - job: test
@@ -69,7 +70,7 @@ jobs:
           dotnet tool restore
         displayName: Restore .NET tools
       - pwsh: |
-          dotnet cake --target=Test --configuration="$(CONFIGURATION)" --packageVersion="$(PACKAGE_VERSION)" --prerelease="$(BUILD_PRERELEASE)" --pullrequest="$env:SYSTEM_PULLREQUEST_PULLREQUESTNUMBER"
+          dotnet cake --target=Test --configuration="$(CONFIGURATION)" --packageVersion="$(PACKAGE_VERSION)" --prerelease="$(BUILD_PRERELEASE)" --pullrequest="$(PR_NUMBER)"
         displayName: Run tests
       - task: PublishBuildArtifacts@1
         displayName: Publish artifacts
diff --git a/build.cake b/build.cake
index d7eedea..7c834c2 100644
--- a/build.cake
+++ b/build.cake
@@ -1,7 +1,7 @@
 var target = Argument("target", "Default");
 var configuration = Argument("configuration", "Release");
-var pullrequest = Argument("pullrequest", "");
-var prerelease = Argument("prerelease", true) || !string.IsNullOrEmpty(pullrequest);
+var pullrequest = Argument("pullrequest", 0);
+var prerelease = Argument("prerelease", true) || pullrequest > 0;
 
 // a bit of logic to create the version number:
 //  - input                     = 1.2.3.4
@@ -13,7 +13,7 @@ var assemblyVersion = $"{version.Major}.0.0.0";
 var fileVersion     = $"{version.Major}.{version.Minor}.{version.Build}.0";
 var infoVersion     = $"{version.Major}.{version.Minor}.{version.Build}.{version.Revision}";
 var packageVersion  = $"{version.Major}.{version.Minor}.{version.Build}";
-var previewlabel = string.IsNullOrEmpty(pullrequest) ? "preview" : $"pr.{pullrequest}";
+var previewlabel = pullrequest > 0 ? $"pr.{pullrequest}" : "preview";
 var previewVersion  = $"{packageVersion}-{previewlabel}.{previewNumber}";
 var finalBuildNumber = prerelease ? previewVersion : packageVersion;