diff --git a/DevProxy.Integration.Tests/ApiCenterClientTests.cs b/DevProxy.Integration.Tests/ApiCenterClientTests.cs
index 7c27ec1b..b561ef53 100644
--- a/DevProxy.Integration.Tests/ApiCenterClientTests.cs
+++ b/DevProxy.Integration.Tests/ApiCenterClientTests.cs
@@ -10,6 +10,8 @@ namespace DevProxy.Integration.Tests;
 
 public sealed class ApiCenterClientTests
 {
+    private const string VariableName = "DEVPROXY_TEST_APICENTER_SUBSCRIPTION_ID";
+
     [Theory]
     [InlineData("subscriptionId")]
     [InlineData("resourceGroupName")]
@@ -30,4 +32,42 @@ public void Constructor_MissingSetting_ErrorUsesConfigKeyName(string missingKey)
 
         Assert.Equal($"Specify {missingKey} in the configuration.", ex.Message);
     }
+
+    [Fact]
+    public void Constructor_UnsetEnvironmentVariable_Throws()
+    {
+        Environment.SetEnvironmentVariable(VariableName, null);
+
+        var ex = Assert.Throws<ArgumentException>(() => CreateClient($"@{VariableName}"));
+
+        Assert.Contains(VariableName, ex.Message, StringComparison.Ordinal);
+        Assert.Contains("subscriptionId", ex.Message, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void Constructor_SetEnvironmentVariable_UsesItsValue()
+    {
+        Environment.SetEnvironmentVariable(VariableName, "00000000-0000-0000-0000-000000000000");
+        try
+        {
+            var configuration = NewConfiguration($"@{VariableName}");
+            using var client = new ApiCenterClient(configuration, NullLogger<ApiCenterClient>.Instance);
+
+            Assert.Equal("00000000-0000-0000-0000-000000000000", configuration.SubscriptionId);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(VariableName, null);
+        }
+    }
+
+    private static ApiCenterClient CreateClient(string subscriptionId) =>
+        new(NewConfiguration(subscriptionId), NullLogger<ApiCenterClient>.Instance);
+
+    private static ApiCenterClientConfiguration NewConfiguration(string subscriptionId) => new()
+    {
+        SubscriptionId = subscriptionId,
+        ResourceGroupName = "rg",
+        ServiceName = "svc"
+    };
 }
diff --git a/DevProxy.Plugins/ApiCenter/ApiCenterClient.cs b/DevProxy.Plugins/ApiCenter/ApiCenterClient.cs
index f267482d..7cb5fd02 100644
--- a/DevProxy.Plugins/ApiCenter/ApiCenterClient.cs
+++ b/DevProxy.Plugins/ApiCenter/ApiCenterClient.cs
@@ -56,22 +56,10 @@ internal ApiCenterClient(ApiCenterClientConfiguration configuration, ILogger<Api
         }
 
         // load configuration from env vars
-        if (configuration.SubscriptionId.StartsWith('@'))
-        {
-            configuration.SubscriptionId = Environment.GetEnvironmentVariable(configuration.SubscriptionId[1..]) ?? configuration.SubscriptionId;
-        }
-        if (configuration.ResourceGroupName.StartsWith('@'))
-        {
-            configuration.ResourceGroupName = Environment.GetEnvironmentVariable(configuration.ResourceGroupName[1..]) ?? configuration.ResourceGroupName;
-        }
-        if (configuration.ServiceName.StartsWith('@'))
-        {
-            configuration.ServiceName = Environment.GetEnvironmentVariable(configuration.ServiceName[1..]) ?? configuration.ServiceName;
-        }
-        if (configuration.WorkspaceName.StartsWith('@'))
-        {
-            configuration.WorkspaceName = Environment.GetEnvironmentVariable(configuration.WorkspaceName[1..]) ?? configuration.WorkspaceName;
-        }
+        configuration.SubscriptionId = ResolveEnvironmentVariable(configuration.SubscriptionId, "subscriptionId");
+        configuration.ResourceGroupName = ResolveEnvironmentVariable(configuration.ResourceGroupName, "resourceGroupName");
+        configuration.ServiceName = ResolveEnvironmentVariable(configuration.ServiceName, "serviceName");
+        configuration.WorkspaceName = ResolveEnvironmentVariable(configuration.WorkspaceName, "workspaceName");
 
         _configuration = configuration;
         _logger = logger;
@@ -91,6 +79,20 @@ internal ApiCenterClient(ApiCenterClientConfiguration configuration, ILogger<Api
         }
     }
 
+    private static string ResolveEnvironmentVariable(string value, string settingName)
+    {
+        if (!value.StartsWith('@'))
+        {
+            return value;
+        }
+
+        var variableName = value[1..];
+        var variableValue = Environment.GetEnvironmentVariable(variableName);
+        return string.IsNullOrEmpty(variableValue)
+            ? throw new ArgumentException($"Environment variable {variableName} referenced by {settingName} isn't set.")
+            : variableValue;
+    }
+
     internal Task<string?> GetAccessTokenAsync(CancellationToken cancellationToken) =>
         _authenticationHandler.GetAccessTokenAsync(cancellationToken);