diff --git a/um_wasm/src/exports/ncm.rs b/um_wasm/src/exports/ncm.rs
index 4e41773..5bc49ac 100644
--- a/um_wasm/src/exports/ncm.rs
+++ b/um_wasm/src/exports/ncm.rs
@@ -19,22 +19,25 @@ impl JsNCMFile {
 
     /// Open NCM file.
     /// If everything is ok, return `0`.
+    /// If it needs more header bytes, return positive integer.
+    /// If it was not a valid NCM file, return -1.
     ///
     /// # Arguments
     ///
     /// * `header`: Header bytes of NCM file.
     ///
-    /// returns: Result<usize, JsError>
+    /// returns: Result<i32, JsError>
     ///
     /// If it needs more bytes, the new header size will be returned.
     /// If the header was large enough, it will return 0.
-    pub fn open(&mut self, header: &[u8]) -> Result<usize, JsError> {
+    pub fn open(&mut self, header: &[u8]) -> Result<i32, JsError> {
         match NCMFile::new(header) {
             Ok(ncm) => {
                 self.ncm = Some(ncm);
                 Ok(0)
             }
-            Err(NetEaseCryptoError::HeaderTooSmall(n)) => Ok(n),
+            Err(NetEaseCryptoError::HeaderTooSmall(n)) => Ok(n as i32),
+            Err(NetEaseCryptoError::NotNCMFile) => Ok(-1),
             Err(err) => Err(JsError::new(err.to_string().as_str())),
         }
     }