diff --git a/pro/src/sync.ts b/pro/src/sync.ts
index c2e4f4e..4599fc3 100644
--- a/pro/src/sync.ts
+++ b/pro/src/sync.ts
@@ -28,6 +28,7 @@ import {
 } from "../../src/metadataOnRemote";
 import {
   atWhichLevel,
+  checkValidName,
   getParentFolder,
   isHiddenPath,
   isSpecialFolderNameToSkip,
@@ -211,6 +212,13 @@ const ensembleMixedEnties = async (
       continue;
     }
 
+    const checkValidNameResult = checkValidName(key);
+    if (!checkValidNameResult.result) {
+      throw Error(
+        `your remote folder/file name is invalid: ${checkValidNameResult.reason}`
+      );
+    }
+
     finalMappings[key] = {
       key: key,
       remote: remoteCopied,
@@ -280,6 +288,13 @@ const ensembleMixedEnties = async (
       continue;
     }
 
+    const checkValidNameResult = checkValidName(key);
+    if (!checkValidNameResult.result) {
+      throw Error(
+        `your local folder/file name is invalid: ${checkValidNameResult.reason}`
+      );
+    }
+
     // TODO: abstraction leaking?
     const localCopied = await fsEncrypt.encryptEntity(
       copyEntityAndFixTimeFormat(local, serviceType)
@@ -920,6 +935,8 @@ const getSyncPlanInplace = async (
   keptFolder.delete("/");
   keptFolder.delete("");
   if (keptFolder.size > 0) {
+    console.error(sortedKeys);
+    console.error(mixedEntityMappings);
     throw Error(`unexpectedly keptFolder no decisions: ${[...keptFolder]}`);
   }
 
diff --git a/src/misc.ts b/src/misc.ts
index a0faba2..c1b61c4 100644
--- a/src/misc.ts
+++ b/src/misc.ts
@@ -742,3 +742,110 @@ export const getSha1 = async (x: ArrayBuffer, stringify: "base64" | "hex") => {
   }
   throw Error(`not supported stringify option = ${stringify}`);
 };
+
+/**
+ * https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file#naming-conventions
+ * https://support.microsoft.com/en-us/office/restrictions-and-limitations-in-onedrive-and-sharepoint-64883a5d-228e-48f5-b3d2-eb39e07630fa#invalidcharacters
+ */
+export const checkValidName = (x: string) => {
+  if (x === undefined || x === "") {
+    // what??
+    return {
+      reason: "empty",
+      result: false,
+    };
+  }
+
+  // The following reserved characters:
+  const invalidChars = '*"<>:|?'.split("");
+  for (const c of invalidChars) {
+    if (x.includes(c)) {
+      return {
+        reason: `reserved character: ${c}`,
+        result: false,
+      };
+    }
+  }
+
+  // directory component
+  for (const c of [".", ".."]) {
+    if (
+      x === c ||
+      x.endsWith(`/${c}`) ||
+      x.startsWith(`${c}/`) ||
+      x.includes(`/${c}/`)
+    ) {
+      return {
+        reason: `directory being ${c}`,
+        result: false,
+      };
+    }
+  }
+
+  // reserved file names
+  const reservedNames = [
+    "CON",
+    "PRN",
+    "AUX",
+    "NUL",
+    "COM0",
+    "COM1",
+    "COM2",
+    "COM3",
+    "COM4",
+    "COM5",
+    "COM6",
+    "COM7",
+    "COM8",
+    "COM9",
+    "COM¹",
+    "COM²",
+    "COM³",
+    "LPT0",
+    "LPT1",
+    "LPT2",
+    "LPT3",
+    "LPT4",
+    "LPT5",
+    "LPT6",
+    "LPT7",
+    "LPT8",
+    "LPT9",
+    "LPT¹",
+    "LPT²",
+    "LPT³",
+  ];
+  for (const f of reservedNames) {
+    if (
+      x === f ||
+      x.startsWith(`${f}.`) ||
+      x.startsWith(`${f}/`) ||
+      x.includes(`/${f}/`) ||
+      x.endsWith(`/${f}`) ||
+      x.includes(`/${f}.`)
+    ) {
+      return {
+        reason: `reserved folder/file name: ${f}`,
+        result: false,
+      };
+    }
+  }
+
+  // Do not end a file or directory name with a space or a period.
+  if (
+    x.endsWith(" ") ||
+    x.endsWith(".") ||
+    x.includes(" /") ||
+    x.includes("./")
+  ) {
+    return {
+      reason: `folder/file name ending with a space or a period`,
+      result: false,
+    };
+  }
+
+  return {
+    reason: "ok",
+    result: true,
+  };
+};
diff --git a/tests/misc.test.ts b/tests/misc.test.ts
index 1379e0c..9e2a564 100644
--- a/tests/misc.test.ts
+++ b/tests/misc.test.ts
@@ -363,6 +363,82 @@ describe("Misc: split chunk ranges", () => {
   });
 });
 
+describe("Misc: check valid file names", () => {
+  it("should be ok for normal file nmes", () => {
+    let item = "what/.hidden/what/what/what";
+    assert.ok(misc.checkValidName(item).result);
+
+    item = "ssss/%%%^^^$xxxx.md";
+    assert.ok(misc.checkValidName(item).result);
+  });
+
+  it("should be not ok for reserved characters", () => {
+    let item = "a**";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "a?*";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "<>:";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "<>:/ssss";
+    assert.ok(!misc.checkValidName(item).result);
+  });
+
+  it("should be not ok for reserved names", () => {
+    let item = "CON";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "CON.txt";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "CON.md";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "con"; // lower case is ok
+    assert.ok(misc.checkValidName(item).result);
+
+    item = "CON/";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "CON.dir/";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "CON.dir.folder/";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx/CON";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx/CON.txt";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx/CON.txt.md";
+    assert.ok(!misc.checkValidName(item).result);
+  });
+
+  it("should be not ok for invalid endings", () => {
+    let item = "xxx ";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "/xxx ";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx.yyy.";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx.yyy.";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx /";
+    assert.ok(!misc.checkValidName(item).result);
+
+    item = "xxx.yyy./";
+    assert.ok(!misc.checkValidName(item).result);
+  });
+});
+
 describe("Misc: Dropbox: should fix the folder name cases", () => {
   it("should do nothing on empty folders", () => {
     const input: any[] = [];