diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index ac4faa72e97bc..5180c765990bb 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -6613,7 +6613,7 @@ namespace ts {
         realpath?(path: string): string;
         getCurrentDirectory?(): string;
         getDirectories?(path: string): string[];
-        useCaseSensitiveFileNames?: boolean | (() => boolean);
+        useCaseSensitiveFileNames?: boolean | (() => boolean) | undefined;
     }
 
     /**
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index a5bd8005936da..680978e77c2a5 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -3164,7 +3164,7 @@ declare namespace ts {
         realpath?(path: string): string;
         getCurrentDirectory?(): string;
         getDirectories?(path: string): string[];
-        useCaseSensitiveFileNames?: boolean | (() => boolean);
+        useCaseSensitiveFileNames?: boolean | (() => boolean) | undefined;
     }
     /**
      * Used by services to specify the minimum host area required to set up source files under any compilation settings
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 01ed44a4dfe9b..1a7f94807720a 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -3164,7 +3164,7 @@ declare namespace ts {
         realpath?(path: string): string;
         getCurrentDirectory?(): string;
         getDirectories?(path: string): string[];
-        useCaseSensitiveFileNames?: boolean | (() => boolean);
+        useCaseSensitiveFileNames?: boolean | (() => boolean) | undefined;
     }
     /**
      * Used by services to specify the minimum host area required to set up source files under any compilation settings