diff --git a/src/GitVersionCore.Tests/BuildServers/VsoAgentTests.cs b/src/GitVersionCore.Tests/BuildServers/VsoAgentTests.cs
index f78abd3476..b265d7db3e 100644
--- a/src/GitVersionCore.Tests/BuildServers/VsoAgentTests.cs
+++ b/src/GitVersionCore.Tests/BuildServers/VsoAgentTests.cs
@@ -40,4 +40,15 @@ public void EscapeValues()
         vsVersion[0].ShouldBe("##vso[task.setvariable variable=GitVersion.Foo;]0.8.0-unstable568 Branch:'develop' Sha:'ee69bff1087ebc95c6b43aa2124bd58f5722e0cb'");
     }
 
+    [Test]
+    public void MissingEnvShouldNotBlowUp()
+    {
+        Environment.SetEnvironmentVariable(key, null, EnvironmentVariableTarget.Process);
+
+        var versionBuilder = new VsoAgent();
+        var semver = "0.0.0-Unstable4";
+        var vars = new TestableVersionVariables(fullSemVer: semver);
+        var vsVersion = versionBuilder.GenerateSetVersionMessage(vars);
+        vsVersion.ShouldBe(semver);
+    }
 }
\ No newline at end of file
diff --git a/src/GitVersionCore/BuildServers/VsoAgent.cs b/src/GitVersionCore/BuildServers/VsoAgent.cs
index 67494b9b03..cb2b63d7ea 100644
--- a/src/GitVersionCore/BuildServers/VsoAgent.cs
+++ b/src/GitVersionCore/BuildServers/VsoAgent.cs
@@ -1,6 +1,7 @@
 namespace GitVersion
 {
     using System;
+    using System.Collections.Generic;
     using System.Linq;
     using System.Text.RegularExpressions;
 
@@ -35,19 +36,30 @@ public override string GenerateSetVersionMessage(VersionVariables variables)
         {
             // For VSO, we'll get the Build Number and insert GitVersion variables where
             // specified
-            var buildNum = Environment.GetEnvironmentVariable("BUILD_BUILDNUMBER");
+            var buildNumberEnv = Environment.GetEnvironmentVariable("BUILD_BUILDNUMBER");
+            if (String.IsNullOrWhiteSpace(buildNumberEnv))
+                return variables.FullSemVer;
 
-            var newBuildNum = variables.Aggregate(buildNum, (current, kvp) =>
-                current.RegexReplace(string.Format(@"\$\(GITVERSION[_\.]{0}\)", kvp.Key), kvp.Value ?? string.Empty, RegexOptions.IgnoreCase));
+            var newBuildNumber = variables.Aggregate(buildNumberEnv, ReplaceVariables);
 
             // If no variable substitution has happened, use FullSemVer
-            if (buildNum == newBuildNum)
+            if (buildNumberEnv == newBuildNumber)
             {
-                var buildNumber = variables.FullSemVer.EndsWith("+0") ? variables.FullSemVer.Substring(0, variables.FullSemVer.Length - 2) : variables.FullSemVer;
-                return string.Format("##vso[build.updatebuildnumber]{0}", buildNumber);
+                var buildNumber = variables.FullSemVer.EndsWith("+0")
+                                ? variables.FullSemVer.Substring(0, variables.FullSemVer.Length - 2)
+                                : variables.FullSemVer;
+                
+                return $"##vso[build.updatebuildnumber]{buildNumber}";
             }
 
-            return string.Format("##vso[build.updatebuildnumber]{0}", newBuildNum);
+            return $"##vso[build.updatebuildnumber]{newBuildNumber}";
+        }
+
+        private static string ReplaceVariables(string buildNumberEnv, KeyValuePair<string, string> variable)
+        {
+            var pattern = $@"\$\(GITVERSION[_\.]{variable.Key}\)";
+            var replacement = variable.Value ?? string.Empty;
+            return buildNumberEnv.RegexReplace(pattern, replacement, RegexOptions.IgnoreCase);
         }
     }
 }