diff --git a/crates/rust-analyzer/src/main_loop.rs b/crates/rust-analyzer/src/main_loop.rs
index 5480b9e4d41..221f464b65b 100644
--- a/crates/rust-analyzer/src/main_loop.rs
+++ b/crates/rust-analyzer/src/main_loop.rs
@@ -44,6 +44,8 @@ pub struct LspError {
 }
 
 impl LspError {
+    pub const UNKNOWN_FILE: i32 = -32900;
+
     pub fn new(code: i32, message: String) -> LspError {
         LspError { code, message }
     }
@@ -805,7 +807,14 @@ fn result_to_task<R>(id: RequestId, result: Result<R::Result>) -> Task
     let response = match result {
         Ok(resp) => Response::new_ok(id, &resp),
         Err(e) => match e.downcast::<LspError>() {
-            Ok(lsp_error) => Response::new_err(id, lsp_error.code, lsp_error.message),
+            Ok(lsp_error) => {
+                if lsp_error.code == LspError::UNKNOWN_FILE {
+                    // Work-around for https://github.com/rust-analyzer/rust-analyzer/issues/1521
+                    Response::new_ok(id, ())
+                } else {
+                    Response::new_err(id, lsp_error.code, lsp_error.message)
+                }
+            }
             Err(e) => {
                 if is_canceled(&e) {
                     Response::new_err(
diff --git a/crates/rust-analyzer/src/world.rs b/crates/rust-analyzer/src/world.rs
index 96efab844d5..dce243ede55 100644
--- a/crates/rust-analyzer/src/world.rs
+++ b/crates/rust-analyzer/src/world.rs
@@ -9,7 +9,6 @@
 };
 
 use crossbeam_channel::{unbounded, Receiver};
-use lsp_server::ErrorCode;
 use lsp_types::Url;
 use parking_lot::RwLock;
 use ra_cargo_watch::{url_from_path_with_drive_lowercasing, CheckOptions, CheckWatcher};
@@ -252,8 +251,9 @@ pub fn uri_to_file_id(&self, uri: &Url) -> Result<FileId> {
         let path = uri.to_file_path().map_err(|()| format!("invalid uri: {}", uri))?;
         let file = self.vfs.read().path2file(&path).ok_or_else(|| {
             // Show warning as this file is outside current workspace
+            // FIXME: just handle such files, and remove `LspError::UNKNOWN_FILE`.
             LspError {
-                code: ErrorCode::InvalidRequest as i32,
+                code: LspError::UNKNOWN_FILE,
                 message: "Rust file outside current workspace is not supported yet.".to_string(),
             }
         })?;