diff --git a/tests/xharness/Jenkins.cs b/tests/xharness/Jenkins.cs
index 2d5287011f04..d2fd8c4f142f 100644
--- a/tests/xharness/Jenkins.cs
+++ b/tests/xharness/Jenkins.cs
@@ -2416,7 +2416,7 @@ protected void SetEnvironmentVariables (Process process)
 				process.StartInfo.EnvironmentVariables ["MD_APPLE_SDK_ROOT"] = Harness.XcodeRoot;
 				process.StartInfo.EnvironmentVariables ["MD_MTOUCH_SDK_ROOT"] = Path.Combine (Harness.IOS_DESTDIR, "Library", "Frameworks", "Xamarin.iOS.framework", "Versions", "Current");
 				process.StartInfo.EnvironmentVariables ["TargetFrameworkFallbackSearchPaths"] = Path.Combine (Harness.IOS_DESTDIR, "Library", "Frameworks", "Mono.framework", "External", "xbuild-frameworks");
-				process.StartInfo.EnvironmentVariables ["MSBuildExtensionsPath"] = Path.Combine (Harness.IOS_DESTDIR, "Library", "Frameworks", "Mono.framework", "External", "xbuild");
+				process.StartInfo.EnvironmentVariables ["MSBuildExtensionsPathFallbackPathsOverride"] = Path.Combine (Harness.IOS_DESTDIR, "Library", "Frameworks", "Mono.framework", "External", "xbuild");
 				break;
 			case TestPlatform.Mac:
 			case TestPlatform.Mac_Classic:
@@ -2426,7 +2426,7 @@ protected void SetEnvironmentVariables (Process process)
 			case TestPlatform.Mac_UnifiedXM45_32:
 				process.StartInfo.EnvironmentVariables ["MD_APPLE_SDK_ROOT"] = Harness.XcodeRoot;
 				process.StartInfo.EnvironmentVariables ["TargetFrameworkFallbackSearchPaths"] = Path.Combine (Harness.MAC_DESTDIR, "Library", "Frameworks", "Mono.framework", "External", "xbuild-frameworks");
-				process.StartInfo.EnvironmentVariables ["MSBuildExtensionsPath"] = Path.Combine (Harness.MAC_DESTDIR, "Library", "Frameworks", "Mono.framework", "External", "xbuild");
+				process.StartInfo.EnvironmentVariables ["MSBuildExtensionsPathFallbackPathsOverride"] = Path.Combine (Harness.MAC_DESTDIR, "Library", "Frameworks", "Mono.framework", "External", "xbuild");
 				process.StartInfo.EnvironmentVariables ["XamarinMacFrameworkRoot"] = Path.Combine (Harness.MAC_DESTDIR, "Library", "Frameworks", "Xamarin.Mac.framework", "Versions", "Current");
 				process.StartInfo.EnvironmentVariables ["XAMMAC_FRAMEWORK_PATH"] = Path.Combine (Harness.MAC_DESTDIR, "Library", "Frameworks", "Xamarin.Mac.framework", "Versions", "Current");
 				break;