diff --git a/CHANGELOG.md b/CHANGELOG.md
index 37304fb3..0d7a9d1c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,8 @@ Note: In this file, do not use the hard wrap in the middle of a sentence for com
 
 ## [Unreleased]
 
+- [Fix mapping error in `--doctests`.](https://github.com/taiki-e/cargo-llvm-cov/pull/40)
+
 ## [0.1.0-alpha.4] - 2021-06-13
 
 - [cargo-llvm-cov no longer requires rustfilt.](https://github.com/taiki-e/cargo-llvm-cov/pull/29)
diff --git a/src/main.rs b/src/main.rs
index e894bf85..4a784031 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -102,6 +102,11 @@ fn main() -> Result<()> {
         }
 
         cargo.args(&["test", "--target-dir"]).arg(&cx.target_dir);
+        if cx.doctests && !cx.unstable_flags.iter().any(|f| f == "doctest-in-workspace") {
+            // https://github.com/rust-lang/cargo/issues/9427
+            cargo.arg("-Z");
+            cargo.arg("doctest-in-workspace");
+        }
         append_args(cx, &mut cargo);
 
         cargo.stdout_to_stderr().run()?;