diff --git a/src/body.rs b/src/body.rs
index 5ec8526..a7a2d97 100644
--- a/src/body.rs
+++ b/src/body.rs
@@ -43,11 +43,21 @@ impl Default for Payload {
     }
 }
 
+/// The size of the body.
+///
+/// *Internal API*
+#[derive(Debug)]
+pub(crate) enum BodySize {
+    Empty,
+    Unknown,
+    Known(u64),
+}
+
 /// Payloads are turned into this type where we can hold both a size and the reader.
 ///
 /// *Internal API*
 pub(crate) struct SizedReader {
-    pub size: Option<usize>,
+    pub size: BodySize,
     pub reader: Box<dyn Read + 'static>,
 }
 
@@ -58,7 +68,7 @@ impl fmt::Debug for SizedReader {
 }
 
 impl SizedReader {
-    fn new(size: Option<usize>, reader: Box<dyn Read + 'static>) -> Self {
+    fn new(size: BodySize, reader: Box<dyn Read + 'static>) -> Self {
         SizedReader { size, reader }
     }
 }
@@ -66,7 +76,7 @@ impl SizedReader {
 impl Payload {
     pub fn into_read(self) -> SizedReader {
         match self {
-            Payload::Empty => SizedReader::new(Some(0), Box::new(empty())),
+            Payload::Empty => SizedReader::new(BodySize::Empty, Box::new(empty())),
             Payload::Text(text, _charset) => {
                 #[cfg(feature = "charset")]
                 let bytes = {
@@ -79,20 +89,20 @@ impl Payload {
                 let bytes = text.into_bytes();
                 let len = bytes.len();
                 let cursor = Cursor::new(bytes);
-                SizedReader::new(Some(len), Box::new(cursor))
+                SizedReader::new(BodySize::Known(len as u64), Box::new(cursor))
             }
             #[cfg(feature = "json")]
             Payload::JSON(v) => {
                 let bytes = serde_json::to_vec(&v).expect("Bad JSON in payload");
                 let len = bytes.len();
                 let cursor = Cursor::new(bytes);
-                SizedReader::new(Some(len), Box::new(cursor))
+                SizedReader::new(BodySize::Known(len as u64), Box::new(cursor))
             }
-            Payload::Reader(read) => SizedReader::new(None, read),
+            Payload::Reader(read) => SizedReader::new(BodySize::Unknown, read),
             Payload::Bytes(bytes) => {
                 let len = bytes.len();
                 let cursor = Cursor::new(bytes);
-                SizedReader::new(Some(len), Box::new(cursor))
+                SizedReader::new(BodySize::Known(len as u64), Box::new(cursor))
             }
         }
     }
diff --git a/src/request.rs b/src/request.rs
index 12848bd..31478cc 100644
--- a/src/request.rs
+++ b/src/request.rs
@@ -8,6 +8,7 @@ use qstring::QString;
 use url::{form_urlencoded, Url};
 
 use crate::agent::{self, Agent, AgentState};
+use crate::body::BodySize;
 use crate::body::{Payload, SizedReader};
 use crate::error::Error;
 use crate::header::{self, Header};
@@ -604,8 +605,14 @@ impl Request {
         // Sized bodies are retryable only if they are zero-length because of
         // coincidences of the current implementation - the function responsible
         // for retries doesn't have a way to replay a Payload.
-        let no_body = body.size.is_none() || body.size.unwrap() > 0;
-        idempotent && no_body
+        let retryable_body = match body.size {
+            BodySize::Unknown => false,
+            BodySize::Known(0) => true,
+            BodySize::Known(_) => false,
+            BodySize::Empty => true,
+        };
+
+        idempotent && retryable_body
     }
 }
 
diff --git a/src/unit.rs b/src/unit.rs
index 1f5d9fc..4616f28 100644
--- a/src/unit.rs
+++ b/src/unit.rs
@@ -8,7 +8,7 @@ use url::Url;
 use cookie::{Cookie, CookieJar};
 
 use crate::agent::AgentState;
-use crate::body::{self, Payload, SizedReader};
+use crate::body::{self, BodySize, Payload, SizedReader};
 use crate::header;
 use crate::resolve::ArcResolver;
 use crate::stream::{self, connect_test, Stream};
@@ -56,15 +56,17 @@ impl Unit {
                 // Content-Length,
                 // otherwise, use the chunked Transfer-Encoding (only if no other Transfer-Encoding
                 // has been set
-                if let Some(size) = body.size {
-                    if size != 0 {
-                        extra.push(Header::new("Content-Length", &format!("{}", size)));
+                match body.size {
+                    BodySize::Known(size) => {
+                        extra.push(Header::new("Content-Length", &format!("{}", size)))
                     }
-                } else {
-                    if !is_transfer_encoding_set {
-                        extra.push(Header::new("Transfer-Encoding", "chunked"));
-                        is_chunked = true;
+                    BodySize::Unknown => {
+                        if !is_transfer_encoding_set {
+                            extra.push(Header::new("Transfer-Encoding", "chunked"));
+                            is_chunked = true;
+                        }
                     }
+                    BodySize::Empty => {}
                 }
             }