diff --git a/src/cli.rs b/src/cli.rs
index 9f8abfc..1c91c0c 100644
--- a/src/cli.rs
+++ b/src/cli.rs
@@ -92,7 +92,7 @@ pub fn run() -> Result<()> {
         },
         Some(Command::Done) => match opt.entry {
             None => return Err(Error::NoNumber),
-            Some(n) => done::mark_done(&mut files, &entries, &layout, n, now)?,
+            Some(n) => done::mark_done(&mut files, &entries, &layout, &[n], now)?,
         },
         Some(Command::Fmt) => files.mark_all_dirty(),
     }
diff --git a/src/cli/done.rs b/src/cli/done.rs
index e24505b..7c32fff 100644
--- a/src/cli/done.rs
+++ b/src/cli/done.rs
@@ -1,24 +1,36 @@
+use std::vec;
+
 use chrono::NaiveDateTime;
 
 use crate::eval::Entry;
 use crate::files::commands::Done;
 use crate::files::Files;
 
-use super::error::Result;
+use super::error::{Error, Result};
 use super::layout::line::LineLayout;
 
 pub fn mark_done(
     files: &mut Files,
     entries: &[Entry],
     layout: &LineLayout,
-    number: usize,
+    numbers: &[usize],
     now: NaiveDateTime,
 ) -> Result<()> {
-    let entry = &entries[layout.look_up_number(number)?];
-    let done = Done {
-        date: entry.dates.map(|dates| dates.into()),
-        done_at: now.date(),
-    };
-    files.add_done(number, entry.source, done)?;
-    Ok(())
+    let mut not_tasks = vec![];
+    for &number in numbers {
+        let entry = &entries[layout.look_up_number(number)?];
+        let done = Done {
+            date: entry.dates.map(|dates| dates.into()),
+            done_at: now.date(),
+        };
+        if !files.add_done(entry.source, done) {
+            not_tasks.push(number);
+        }
+    }
+
+    if not_tasks.is_empty() {
+        Ok(())
+    } else {
+        Err(Error::NotATask(not_tasks))
+    }
 }
diff --git a/src/cli/error.rs b/src/cli/error.rs
index 95bb5ec..e0d19e4 100644
--- a/src/cli/error.rs
+++ b/src/cli/error.rs
@@ -12,6 +12,8 @@ pub enum Error {
     NoNumber,
     #[error("No entry with number {0}")]
     NoSuchEntry(usize),
+    #[error("Not a task")]
+    NotATask(Vec<usize>),
 }
 
 pub type Result<T> = result::Result<T, Error>;
diff --git a/src/files.rs b/src/files.rs
index 35fe842..93ec56e 100644
--- a/src/files.rs
+++ b/src/files.rs
@@ -163,14 +163,19 @@ impl Files {
         &self.files[source.file].file.commands[source.command]
     }
 
-    pub fn add_done(&mut self, number: usize, source: Source, done: Done) -> Result<()> {
+    /// Add a [`Done`] statement to the task identified by `source`.
+    ///
+    /// Returns whether the addition was successful. It can fail if the entry
+    /// identified by `source` is a note, not a task.
+    #[must_use]
+    pub fn add_done(&mut self, source: Source, done: Done) -> bool {
         let file = &mut self.files[source.file];
         match &mut file.file.commands[source.command] {
             Command::Task(t) => t.done.push(done),
-            Command::Note(_) => return Err(Error::NotATask(vec![number])),
+            Command::Note(_) => return false,
         }
         file.dirty = true;
-        Ok(())
+        true
     }
 
     pub fn commands(&self) -> Vec<SourcedCommand<'_>> {
diff --git a/src/files/error.rs b/src/files/error.rs
index 1076eca..67acee4 100644
--- a/src/files/error.rs
+++ b/src/files/error.rs
@@ -24,8 +24,6 @@ pub enum Error {
         file2: PathBuf,
         tz2: String,
     },
-    #[error("Not a task")]
-    NotATask(Vec<usize>),
 }
 
 impl Error {
@@ -62,16 +60,6 @@ impl Error {
                 eprintln!("  {:?} has time zone {}", file1, tz1);
                 eprintln!("  {:?} has time zone {}", file2, tz2);
             }
-            Error::NotATask(numbers) => {
-                if numbers.is_empty() {
-                    eprintln!("Not a task.");
-                } else if numbers.len() == 1 {
-                    eprintln!("{} is not a task.", numbers[0]);
-                } else {
-                    let numbers = numbers.iter().map(|n| n.to_string()).collect::<Vec<_>>();
-                    eprintln!("{} are not tasks.", numbers.join(", "));
-                }
-            }
         }
     }
 }