diff --git a/src/shims/unix/fs.rs b/src/shims/unix/fs.rs
index 3fdf82ebc6..1420279247 100644
--- a/src/shims/unix/fs.rs
+++ b/src/shims/unix/fs.rs
@@ -1694,7 +1694,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     fn isatty(&mut self, miri_fd: &OpTy<'tcx, Tag>) -> InterpResult<'tcx, i32> {
         let this = self.eval_context_mut();
         #[cfg(unix)]
-        {
+        if matches!(this.machine.isolated_op, IsolatedOp::Allow) {
             let miri_fd = this.read_scalar(miri_fd)?.to_i32()?;
             if let Some(host_fd) =
                 this.machine.file_handler.handles.get(&miri_fd).and_then(|fd| fd.as_unix_host_fd())
@@ -1714,7 +1714,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             }
         }
         // We are attemping to use a Unix interface on a non-Unix platform, or we are on a Unix
-        // platform and the passed file descriptor is not open.
+        // platform and the passed file descriptor is not open, or isolation is enabled
         // FIXME: It should be possible to emulate this at least on Windows by using
         // GetConsoleMode.
         let enotty = this.eval_libc("ENOTTY")?;
diff --git a/tests/pass/libc.rs b/tests/pass/libc.rs
index 6da5279270..ea3848a974 100644
--- a/tests/pass/libc.rs
+++ b/tests/pass/libc.rs
@@ -1,11 +1,12 @@
 //@ignore-windows: No libc on Windows
 //@compile-flags: -Zmiri-disable-isolation
-
 #![feature(rustc_private)]
 
+use std::fs::{remove_file, File};
+use std::os::unix::io::AsRawFd;
+
 extern crate libc;
 
-#[cfg(any(target_os = "linux", target_os = "freebsd"))]
 fn tmp() -> std::path::PathBuf {
     std::env::var("MIRI_TEMP")
         .map(std::path::PathBuf::from)
@@ -15,9 +16,7 @@ fn tmp() -> std::path::PathBuf {
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
 fn test_posix_fadvise() {
     use std::convert::TryInto;
-    use std::fs::{remove_file, File};
     use std::io::Write;
-    use std::os::unix::io::AsRawFd;
 
     let path = tmp().join("miri_test_libc_posix_fadvise.txt");
     // Cleanup before test
@@ -44,9 +43,7 @@ fn test_posix_fadvise() {
 
 #[cfg(any(target_os = "linux"))]
 fn test_sync_file_range() {
-    use std::fs::{remove_file, File};
     use std::io::Write;
-    use std::os::unix::io::AsRawFd;
 
     let path = tmp().join("miri_test_libc_sync_file_range.txt");
     // Cleanup before test.
@@ -319,6 +316,19 @@ fn test_isatty() {
         libc::isatty(libc::STDIN_FILENO);
         libc::isatty(libc::STDOUT_FILENO);
         libc::isatty(libc::STDERR_FILENO);
+
+        // But when we open a file, it is definitely not a TTY.
+        let path = tmp().join("notatty.txt");
+        // Cleanup before test.
+        remove_file(&path).ok();
+        let file = File::create(&path).unwrap();
+
+        assert_eq!(libc::isatty(file.as_raw_fd()), 0);
+        assert_eq!(std::io::Error::last_os_error().raw_os_error().unwrap(), libc::ENOTTY);
+
+        // Cleanup after test.
+        drop(file);
+        remove_file(&path).unwrap();
     }
 }