diff --git a/src/tsserver/server.ts b/src/tsserver/server.ts
index 55612708253..905ccbac0c2 100644
--- a/src/tsserver/server.ts
+++ b/src/tsserver/server.ts
@@ -211,7 +211,7 @@ namespace ts.server {
             if (this.fd >= 0) {
                 const buf = sys.bufferFrom!(s);
                 // eslint-disable-next-line no-null/no-null
-                fs.writeSync(this.fd, buf, 0, buf.length, /*position*/ null!); // TODO: GH#18217
+                fs.writeSync(this.fd, buf as globalThis.Buffer, 0, buf.length, /*position*/ null!); // TODO: GH#18217
             }
             if (this.traceToConsole) {
                 console.warn(s);
@@ -884,7 +884,7 @@ namespace ts.server {
     }
 
     // Override sys.write because fs.writeSync is not reliable on Node 4
-    sys.write = (s: string) => writeMessage(sys.bufferFrom!(s, "utf8"));
+    sys.write = (s: string) => writeMessage(sys.bufferFrom!(s, "utf8")  as globalThis.Buffer);
     sys.watchFile = (fileName, callback) => {
         const watchedFile = pollingWatchedFileSet.addFile(fileName, callback);
         return {