diff --git a/capi/include/hyper.h b/capi/include/hyper.h
index 64e28bf894..2245d6a45d 100644
--- a/capi/include/hyper.h
+++ b/capi/include/hyper.h
@@ -299,6 +299,8 @@ void hyper_body_set_data_func(struct hyper_body *body, hyper_body_data_callback
 
  This makes an owned copy of the bytes, so the `buf` argument can be
  freed or changed afterwards.
+
+ This returns `NULL` if allocating a new buffer fails.
  */
 struct hyper_buf *hyper_buf_copy(const uint8_t *buf, size_t len);
 
diff --git a/src/ffi/body.rs b/src/ffi/body.rs
index 932200b54d..39ba5beffb 100644
--- a/src/ffi/body.rs
+++ b/src/ffi/body.rs
@@ -185,6 +185,8 @@ ffi_fn! {
     ///
     /// This makes an owned copy of the bytes, so the `buf` argument can be
     /// freed or changed afterwards.
+    ///
+    /// This returns `NULL` if allocating a new buffer fails.
     fn hyper_buf_copy(buf: *const u8, len: size_t) -> *mut hyper_buf {
         let slice = unsafe {
             std::slice::from_raw_parts(buf, len)