diff --git a/src/harness/unittests/session.ts b/src/harness/unittests/session.ts
index 74c5de2bbaf..3d9f782b2e0 100644
--- a/src/harness/unittests/session.ts
+++ b/src/harness/unittests/session.ts
@@ -53,6 +53,17 @@ namespace ts.server {
             return new TestSession(opts);
         }
 
+        // Disable sourcemap support for the duration of the test, as sourcemapping the errors generated during this test is slow and not something we care to test
+        let oldPrepare: Function;
+        before(() => {
+            oldPrepare = (Error as any).prepareStackTrace;
+            delete (Error as any).prepareStackTrace;
+        });
+
+        after(() => {
+            (Error as any).prepareStackTrace = oldPrepare;
+        });
+
         beforeEach(() => {
             session = createSession();
             session.send = (msg: protocol.Message) => {
@@ -387,6 +398,18 @@ namespace ts.server {
     });
 
     describe("exceptions", () => {
+
+        // Disable sourcemap support for the duration of the test, as sourcemapping the errors generated during this test is slow and not something we care to test
+        let oldPrepare: Function;
+        before(() => {
+            oldPrepare = (Error as any).prepareStackTrace;
+            delete (Error as any).prepareStackTrace;
+        });
+
+        after(() => {
+            (Error as any).prepareStackTrace = oldPrepare;
+        });
+
         const command = "testhandler";
         class TestSession extends Session {
             lastSent: protocol.Message;