diff --git a/Jakefile.js b/Jakefile.js
index c6b1cde9d4e2b..18f4aa56f8d92 100644
--- a/Jakefile.js
+++ b/Jakefile.js
@@ -24,7 +24,7 @@ const host = process.env.TYPESCRIPT_HOST || process.env.host || "node";
 
 const defaultTestTimeout = 40000;
 const useBuilt =
-    process.env.USE_BUILT === "true" ? true :
+    (process.env.USE_BUILT === "true" || process.env.CI === "true") ? true :
     process.env.LKG === "true" ? false :
     false;
 
diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index c5505ec373669..4bd4801ce8f64 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1396,9 +1396,10 @@ namespace ts {
 
     export function assign<T extends object>(t: T, ...args: (T | undefined)[]) {
         for (const arg of args) {
-            for (const p in arg!) {
-                if (hasProperty(arg!, p)) {
-                    t![p] = arg![p]; // TODO: GH#23368
+            if (arg === undefined) continue;
+            for (const p in arg) {
+                if (hasProperty(arg, p)) {
+                    t[p] = arg[p];
                 }
             }
         }
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index 36c246c999f23..70fccdf87c1a3 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -8347,7 +8347,7 @@ declare namespace ts.server {
         excludedFiles: ReadonlyArray<NormalizedPath>;
         private typeAcquisition;
         updateGraph(): boolean;
-        getExcludedFiles(): ReadonlyArray<NormalizedPath>;
+        getExcludedFiles(): readonly NormalizedPath[];
         getTypeAcquisition(): TypeAcquisition;
         setTypeAcquisition(newTypeAcquisition: TypeAcquisition): void;
     }