diff --git a/src/lib.rs b/src/lib.rs
index bb7588c..cdb2765 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -319,8 +319,9 @@ named!(pub alternative_media_tag<AlternativeMedia>,
 );
 
 named!(pub session_data_tag<SessionData>,
-    do_parse!( tag!("#EXT-X-SESSION-DATA:") >> attributes: key_value_pairs >>
-            ( SessionData::from_hashmap(attributes)))
+    do_parse!( tag!("#EXT-X-SESSION-DATA:") >>
+           session_data: map_res!(key_value_pairs, |attrs| SessionData::from_hashmap(attrs)) >>
+           ( session_data))
 );
 
 named!(pub session_key_tag<SessionKey>,
diff --git a/src/playlist.rs b/src/playlist.rs
index a085e0f..cdecb13 100644
--- a/src/playlist.rs
+++ b/src/playlist.rs
@@ -343,6 +343,9 @@ impl fmt::Display for AlternativeMediaType {
 
 /// [`#EXT-X-SESSION-KEY:<attribute-list>`]
 /// (https://tools.ietf.org/html/draft-pantos-http-live-streaming-19#section-4.3.4.5)
+/// The EXT-X-SESSION-KEY tag allows encryption keys from Media Playlists
+/// to be specified in a Master Playlist.  This allows the client to
+/// preload these keys without having to read the Media Playlist(s) first.
 #[derive(Debug, Default, PartialEq, Clone)]
 pub struct SessionKey(pub Key);
 
@@ -354,34 +357,56 @@ impl SessionKey {
     }
 }
 
+#[derive(Debug, PartialEq, Clone)]
+pub enum SessionDataField {
+    Value(String),
+    Uri(String)
+}
+
 /// [`#EXT-X-SESSION-DATA:<attribute-list>`]
 /// (https://tools.ietf.org/html/draft-pantos-http-live-streaming-19#section-4.3.4.4)
-/// The EXT-X-SESSION-KEY tag allows encryption keys from Media Playlists
-/// to be specified in a Master Playlist.  This allows the client to
-/// preload these keys without having to read the Media Playlist(s) first.
-#[derive(Debug, Default, PartialEq, Clone)]
+/// The EXT-X-SESSION-DATA tag allows arbitrary session data to be carried
+/// in a Master Playlist.
+#[derive(Debug, PartialEq, Clone)]
 pub struct SessionData {
     pub data_id: String,
-    pub value: String,
-    pub uri: String,
+    pub field: SessionDataField,
     pub language: Option<String>,
 }
 
 impl SessionData {
-    pub fn from_hashmap(mut attrs: HashMap<String, String>) -> SessionData {
-        SessionData {
-            data_id: attrs.remove("DATA-ID").unwrap_or_else(String::new),
-            value: attrs.remove("VALUE").unwrap_or_else(String::new),
-            uri: attrs.remove("URI").unwrap_or_else(String::new),
+    pub fn from_hashmap(mut attrs: HashMap<String, String>) -> Result<SessionData, String> {
+        let data_id = match attrs.remove("DATA-ID") {
+            Some(data_id) => data_id,
+            None => return Err("EXT-X-SESSION-DATA field without DATA-ID".to_string())
+        };
+
+        let value = attrs.remove("VALUE");
+        let uri = attrs.remove("URI");
+
+        // SessionData must contain either a VALUE or a URI,
+        // but not both https://tools.ietf.org/html/rfc8216#section-4.3.4.4
+        let field = match (value, uri) {
+            (Some(value), None) => SessionDataField::Value(value),
+            (None, Some(uri)) => SessionDataField::Uri(uri),
+            (Some(_), Some(_)) => return Err(format!["EXT-X-SESSION-DATA tag {} contains both a value and a uri", data_id]),
+            (None, None) => return Err(format!["EXT-X-SESSION-DATA tag {} must contain either a value or a uri", data_id]),
+        };
+
+        Ok(SessionData {
+            data_id,
+            field,
             language: attrs.remove("LANGUAGE"),
-        }
+        })
     }
 
     pub fn write_to<T: Write>(&self, w: &mut T) -> std::io::Result<()> {
         write!(w, "#EXT-X-SESSION-DATA:")?;
         write!(w, "DATA-ID=\"{}\"", self.data_id)?;
-        write!(w, ",VALUE=\"{}\"", self.value)?;
-        write!(w, ",URI=\"{}\"", self.uri)?;
+        match &self.field {
+          SessionDataField::Value(value) => write!(w, ",VALUE=\"{}\"", value)?,
+          SessionDataField::Uri(uri) => write!(w, ",URI=\"{}\"", uri)?,
+        };
         write_some_attribute_quoted!(w, ",LANGUAGE", &self.language)?;
         write!(w, "\n")
     }
diff --git a/tests/lib.rs b/tests/lib.rs
index 41cb456..1b04afe 100644
--- a/tests/lib.rs
+++ b/tests/lib.rs
@@ -333,12 +333,13 @@ fn create_and_parse_master_playlist_full() {
                 closed_captions: Some("closed_captions".into()),
             }
         ],
-        session_data: Some(SessionData {
-            data_id: "****".into(),
-            value: "%%%%".into(),
-            uri: "++++".into(),
-            language: Some("SessionDataLanguage".into()),
-        }),
+        session_data: Some(
+            SessionData {
+                data_id: "****".into(),
+                field: SessionDataField::Value("%%%%".to_string()),
+                language: Some("SessionDataLanguage".into()),
+            }
+        ),
         session_key: Some(SessionKey(Key {
             method: "AES-128".into(),
             uri: Some("https://secure.domain.com".into()),