diff --git a/src/compiler/program.ts b/src/compiler/program.ts
index 0be96b7d55f..0b22d78f35a 100644
--- a/src/compiler/program.ts
+++ b/src/compiler/program.ts
@@ -114,9 +114,9 @@ namespace ts {
                     fileName,
                     data,
                     writeByteOrderMark,
-                    writeFileWorker,
-                    compilerHost.createDirectory || system.createDirectory,
-                    directoryExists);
+                    (f, d, w) => writeFileWorker(f, d, w),
+                    p => (compilerHost.createDirectory || system.createDirectory)(p),
+                    p => directoryExists(p));
 
                 performance.mark("afterIOWrite");
                 performance.measure("I/O Write", "beforeIOWrite", "afterIOWrite");
diff --git a/src/compiler/sys.ts b/src/compiler/sys.ts
index 772eb148fc0..933cea7b729 100644
--- a/src/compiler/sys.ts
+++ b/src/compiler/sys.ts
@@ -533,10 +533,10 @@ namespace ts {
             writeFileEnsuringDirectories(
                 path,
                 data,
-                writeBom,
+                !!writeBom,
                 (p, d, w) => originalWriteFile.call(sys, p, d, w),
-                p => sys.createDirectory.call(sys, p),
-                p => sys.directoryExists.call(sys, p));
+                p => sys.createDirectory(p),
+                p => sys.directoryExists(p));
     }
 
     /*@internal*/
diff --git a/src/compiler/utilities.ts b/src/compiler/utilities.ts
index 8f7602cbe2a..ac29da93899 100644
--- a/src/compiler/utilities.ts
+++ b/src/compiler/utilities.ts
@@ -3714,8 +3714,8 @@ namespace ts {
     export function writeFileEnsuringDirectories(
         path: string,
         data: string,
-        writeByteOrderMark: boolean | undefined,
-        writeFile: (path: string, data: string, writeByteOrderMark?: boolean) => void,
+        writeByteOrderMark: boolean,
+        writeFile: (path: string, data: string, writeByteOrderMark: boolean) => void,
         createDirectory: (path: string) => void,
         directoryExists: (path: string) => boolean): void {
 
diff --git a/src/compiler/watch.ts b/src/compiler/watch.ts
index cc6bd0bcced..eef4a03c07b 100644
--- a/src/compiler/watch.ts
+++ b/src/compiler/watch.ts
@@ -303,7 +303,13 @@ namespace ts {
                 // NOTE: If patchWriteFileEnsuringDirectory has been called,
                 // the host.writeFile will do its own directory creation and
                 // the ensureDirectoriesExist call will always be redundant.
-                writeFileEnsuringDirectories(fileName, text, writeByteOrderMark, host.writeFile!, host.createDirectory!, host.directoryExists!);
+                writeFileEnsuringDirectories(
+                    fileName,
+                    text,
+                    writeByteOrderMark,
+                    (f, w, d) => host.writeFile!(f, w, d),
+                    p => host.createDirectory!(p),
+                    p => host.directoryExists!(p));
 
                 performance.mark("afterIOWrite");
                 performance.measure("I/O Write", "beforeIOWrite", "afterIOWrite");