diff --git a/src/builder.rs b/src/builder.rs
index a26eb31d..4a581ce7 100644
--- a/src/builder.rs
+++ b/src/builder.rs
@@ -120,8 +120,9 @@ impl<W: Write> Builder<W> {
     /// header will be modified.
     ///
     /// Then it will append the header, followed by contents of the stream
-    /// specified by `data`. To produce a valid archive the `size` field of
-    /// `header` must be the same as the length of the stream that's being
+    /// specified by `data`, with the `size` field of `header` enforced as an
+    /// upper limit. To produce a valid archive the `size` field of `header`
+    /// must be less than or equal to the length of the stream that's being
     /// written.
     ///
     /// Note that this will not attempt to seek the archive to a valid position,
@@ -406,9 +407,13 @@ impl<W: Write> Builder<W> {
     }
 }
 
-fn append(mut dst: &mut dyn Write, header: &Header, mut data: &mut dyn Read) -> io::Result<()> {
+fn append(mut dst: &mut dyn Write, header: &Header, data: &mut dyn Read) -> io::Result<()> {
     dst.write_all(header.as_bytes())?;
-    let len = io::copy(&mut data, &mut dst)?;
+    // Guards against there being more data than the header indicates in the case where contents
+    // have been appended to the file since the header was created.
+    let size = header.size()?;
+    let mut limited = data.take(size);
+    let len = io::copy(&mut limited, &mut dst)?;
 
     // Pad with zeros if necessary.
     let buf = [0; 512];
diff --git a/tests/all.rs b/tests/all.rs
index 11103bd6..a5fc97c6 100644
--- a/tests/all.rs
+++ b/tests/all.rs
@@ -1279,10 +1279,12 @@ fn append_long_multibyte() {
     let mut x = tar::Builder::new(Vec::new());
     let mut name = String::new();
     let data: &[u8] = &[];
+    let mut header = Header::new_gnu();
+    header.set_size(data.len() as u64);
     for _ in 0..512 {
         name.push('a');
         name.push('𑢮');
-        x.append_data(&mut Header::new_gnu(), &name, data).unwrap();
+        x.append_data(&mut header, &name, data).unwrap();
         name.pop();
     }
 }
@@ -1385,3 +1387,31 @@ fn header_size_overflow() {
         err
     );
 }
+
+#[test]
+fn file_contents_appended_after_header_creation() {
+    let mut data = String::from("Hello");
+
+    let mut header = Header::new_gnu();
+    header.set_size(data.len() as u64);
+    header.set_cksum();
+
+    // Additional data is appended after header creation
+    data.push_str(", World!");
+
+    let mut ar = Builder::new(Vec::new());
+    t!(ar.append_data(&mut header, "test2", data.as_bytes()));
+
+    let raw = t!(ar.into_inner());
+    let mut ar = Archive::new(Cursor::new(raw));
+    let mut entries = t!(ar.entries());
+    let entry = t!(entries.next().unwrap());
+    let offset = entry.raw_file_position();
+    let mut raw = ar.into_inner();
+
+    let mut s = String::new();
+    raw.set_position(offset);
+    t!(raw.read_to_string(&mut s));
+    let s = s.trim_end_matches(char::from(0));
+    assert_eq!(s, "Hello");
+}