diff --git a/src/frame/data.rs b/src/frame/data.rs
index fbaca6f..b627ce3 100644
--- a/src/frame/data.rs
+++ b/src/frame/data.rs
@@ -3,74 +3,95 @@ use bytes::{BufMut, Bytes, Buf};
 
 use std::fmt;
 
+/// Data frame
+///
+/// Data frames convey arbitrary, variable-length sequences of octets associated
+/// with a stream. One or more DATA frames are used, for instance, to carry HTTP
+/// request or response payloads.
 #[derive(Eq, PartialEq)]
 pub struct Data<T = Bytes> {
     stream_id: StreamId,
     data: T,
-    flags: DataFlag,
+    flags: DataFlags,
     pad_len: Option<u8>,
 }
 
 #[derive(Copy, Clone, Eq, PartialEq)]
-pub struct DataFlag(u8);
+struct DataFlags(u8);
 
 const END_STREAM: u8 = 0x1;
 const PADDED: u8 = 0x8;
 const ALL: u8 = END_STREAM | PADDED;
 
-impl Data<Bytes> {
-    pub fn load(head: Head, mut payload: Bytes) -> Result<Self, Error> {
-        let flags = DataFlag::load(head.flag());
-
-        let pad_len = if flags.is_padded() {
-            let len = try!(util::strip_padding(&mut payload));
-            Some(len)
-        } else {
-            None
-        };
-        Ok(Data {
-            stream_id: head.stream_id(),
-            data: payload,
-            flags: flags,
-            pad_len: pad_len,
-        })
-    }
-}
-
 impl<T> Data<T> {
+    /// Creates a new DATA frame.
+    pub fn new(stream_id: StreamId, payload: T) -> Self {
+        assert!(!stream_id.is_zero());
+
+        Data {
+            stream_id: stream_id,
+            data: payload,
+            flags: DataFlags::default(),
+            pad_len: None,
+        }
+    }
+
+    /// Returns the stream identifer that this frame is associated with.
+    ///
+    /// This cannot be a zero stream identifier.
     pub fn stream_id(&self) -> StreamId {
         self.stream_id
     }
 
+    /// Gets the value of the `END_STREAM` flag for this frame.
+    ///
+    /// If true, this frame is the last that the endpoint will send for the
+    /// identified stream.
+    ///
+    /// Setting this flag causes the stream to enter one of the "half-closed"
+    /// states or the "closed" state (Section 5.1).
     pub fn is_end_stream(&self) -> bool {
         self.flags.is_end_stream()
     }
 
-    pub fn set_end_stream(&mut self) {
-        self.flags.set_end_stream();
-    }
-
-    pub fn unset_end_stream(&mut self) {
-        self.flags.unset_end_stream();
-    }
-
-    pub fn head(&self) -> Head {
-        Head::new(Kind::Data, self.flags.into(), self.stream_id)
+    /// Sets the value for the `END_STREAM` flag on this frame.
+    pub fn set_end_stream(&mut self, val: bool) {
+        if val {
+            self.flags.set_end_stream();
+        } else {
+            self.flags.unset_end_stream();
+        }
     }
 
+    /// Returns a reference to this frame's payload.
+    ///
+    /// This does **not** include any padding that might have been originally
+    /// included.
     pub fn payload(&self) -> &T {
         &self.data
     }
 
+    /// Returns a mutable reference to this frame's payload.
+    ///
+    /// This does **not** include any padding that might have been originally
+    /// included.
     pub fn payload_mut(&mut self) -> &mut T {
         &mut self.data
     }
 
+    /// Consumes `self` and returns the frame's payload.
+    ///
+    /// This does **not** include any padding that might have been originally
+    /// included.
     pub fn into_payload(self) -> T {
         self.data
     }
 
-    pub fn map<F, U>(self, f: F) -> Data<U>
+    pub(crate) fn head(&self) -> Head {
+        Head::new(Kind::Data, self.flags.into(), self.stream_id)
+    }
+
+    pub(crate) fn map<F, U>(self, f: F) -> Data<U>
         where F: FnOnce(T) -> U,
     {
         Data {
@@ -82,23 +103,35 @@ impl<T> Data<T> {
     }
 }
 
-impl<T: Buf> Data<T> {
-    pub fn from_buf(stream_id: StreamId, data: T, eos: bool) -> Self {
-        // TODO ensure that data.remaining() < MAX_FRAME_SIZE
-        let mut flags = DataFlag::default();
-        if eos {
-            flags.set_end_stream();
-        }
-        Data {
-            stream_id,
-            data,
-            flags,
-            pad_len: None,
-        }
-    }
+impl Data<Bytes> {
+    pub(crate) fn load(head: Head, mut payload: Bytes) -> Result<Self, Error> {
+        let flags = DataFlags::load(head.flag());
 
-    pub fn encode_chunk<U: BufMut>(&mut self, dst: &mut U) {
+        // The stream identifier must not be zero
+        if head.stream_id().is_zero() {
+            return Err(Error::InvalidStreamId);
+        }
+
+        let pad_len = if flags.is_padded() {
+            let len = util::strip_padding(&mut payload)?;
+            Some(len)
+        } else {
+            None
+        };
+
+        Ok(Data {
+            stream_id: head.stream_id(),
+            data: payload,
+            flags: flags,
+            pad_len: pad_len,
+        })
+    }
+}
+
+impl<T: Buf> Data<T> {
+    pub(crate) fn encode_chunk<U: BufMut>(&mut self, dst: &mut U) {
         let len = self.data.remaining() as usize;
+
         if len > dst.remaining_mut() {
             unimplemented!();
         }
@@ -125,54 +158,45 @@ impl<T> fmt::Debug for Data<T> {
     }
 }
 
-// ===== impl DataFlag =====
+// ===== impl DataFlags =====
 
-impl DataFlag {
-    pub fn load(bits: u8) -> DataFlag {
-        DataFlag(bits & ALL)
+impl DataFlags {
+    fn load(bits: u8) -> DataFlags {
+        DataFlags(bits & ALL)
     }
 
-    pub fn end_stream() -> DataFlag {
-        DataFlag(END_STREAM)
-    }
-
-    pub fn padded() -> DataFlag {
-        DataFlag(PADDED)
-    }
-
-    pub fn is_end_stream(&self) -> bool {
+    fn is_end_stream(&self) -> bool {
         self.0 & END_STREAM == END_STREAM
     }
 
-    pub fn set_end_stream(&mut self) {
+    fn set_end_stream(&mut self) {
         self.0 |= END_STREAM
     }
 
-    pub fn unset_end_stream(&mut self) {
+    fn unset_end_stream(&mut self) {
         self.0 &= !END_STREAM
     }
 
-    pub fn is_padded(&self) -> bool {
+    fn is_padded(&self) -> bool {
         self.0 & PADDED == PADDED
     }
 }
 
-impl Default for DataFlag {
-    /// Returns a `HeadersFlag` value with `END_HEADERS` set.
+impl Default for DataFlags {
     fn default() -> Self {
-        DataFlag(0)
+        DataFlags(0)
     }
 }
 
-impl From<DataFlag> for u8 {
-    fn from(src: DataFlag) -> u8 {
+impl From<DataFlags> for u8 {
+    fn from(src: DataFlags) -> u8 {
         src.0
     }
 }
 
-impl fmt::Debug for DataFlag {
+impl fmt::Debug for DataFlags {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        let mut f = fmt.debug_struct("DataFlag");
+        let mut f = fmt.debug_struct("DataFlags");
 
         if self.is_end_stream() {
             f.field("end_stream", &true);
diff --git a/src/proto/streams/prioritize.rs b/src/proto/streams/prioritize.rs
index bc878bc..a732625 100644
--- a/src/proto/streams/prioritize.rs
+++ b/src/proto/streams/prioritize.rs
@@ -362,7 +362,7 @@ impl<B, P> Prioritize<B, P>
                 let mut stream = store.resolve(key);
 
                 if eos {
-                    frame.set_end_stream();
+                    frame.set_end_stream(true);
                 }
 
                 self.push_back_frame(frame.into(), &mut stream);
@@ -470,7 +470,7 @@ impl<B, P> Prioritize<B, P>
                             let eos = frame.is_end_stream();
 
                             if frame.payload().remaining() > len {
-                                frame.unset_end_stream();
+                                frame.set_end_stream(false);
                             }
 
                             Frame::Data(frame.map(|buf| {
diff --git a/src/proto/streams/streams.rs b/src/proto/streams/streams.rs
index db7d524..638bc02 100644
--- a/src/proto/streams/streams.rs
+++ b/src/proto/streams/streams.rs
@@ -390,7 +390,7 @@ impl<B, P> StreamRef<B, P>
     where B: Buf,
           P: Peer,
 {
-    pub fn send_data(&mut self, data: B, end_of_stream: bool)
+    pub fn send_data(&mut self, data: B, end_stream: bool)
         -> Result<(), UserError>
     {
         let mut me = self.inner.lock().unwrap();
@@ -399,7 +399,8 @@ impl<B, P> StreamRef<B, P>
         let stream = me.store.resolve(self.key);
 
         // Create the data frame
-        let frame = frame::Data::from_buf(stream.id, data, end_of_stream);
+        let mut frame = frame::Data::new(stream.id, data);
+        frame.set_end_stream(end_stream);
 
         me.actions.transition(stream, |actions, stream| {
             // Send the data frame
diff --git a/tests/codec_read.rs b/tests/codec_read.rs
index 7c19578..0f6f91a 100644
--- a/tests/codec_read.rs
+++ b/tests/codec_read.rs
@@ -46,3 +46,15 @@ fn read_data_padding() {
 
     assert_closed!(codec);
 }
+
+#[test]
+fn read_data_stream_id_zero() {
+    let mut codec = raw_codec! {
+        read => [
+            0, 0, 5, 0, 0, 0, 0, 0, 0,
+            "hello", // Data
+        ];
+    };
+
+    poll_err!(codec);
+}
diff --git a/tests/support/src/codec.rs b/tests/support/src/codec.rs
index 90a0dfe..8d0b4f6 100644
--- a/tests/support/src/codec.rs
+++ b/tests/support/src/codec.rs
@@ -5,6 +5,16 @@ macro_rules! assert_closed {
     }}
 }
 
+#[macro_export]
+macro_rules! poll_err {
+    ($transport:expr) => {{
+        match $transport.poll() {
+            Err(e) => e,
+            frame => panic!("expected error; actual={:?}", frame),
+        }
+    }}
+}
+
 #[macro_export]
 macro_rules! poll_data {
     ($transport:expr) => {{