diff --git a/src/sockref.rs b/src/sockref.rs
index f5807d80..257323be 100644
--- a/src/sockref.rs
+++ b/src/sockref.rs
@@ -109,8 +109,10 @@ where
 {
     /// The caller must ensure `S` is actually a socket.
     fn from(socket: &'s S) -> Self {
+        let fd = socket.as_raw_fd();
+        assert!(fd >= 0);
         SockRef {
-            socket: ManuallyDrop::new(unsafe { Socket::from_raw_fd(socket.as_raw_fd()) }),
+            socket: ManuallyDrop::new(unsafe { Socket::from_raw_fd(fd) }),
             _lifetime: PhantomData,
         }
     }
@@ -125,8 +127,10 @@ where
 {
     /// See the `From<&impl AsRawFd>` implementation.
     fn from(socket: &'s S) -> Self {
+        let socket = socket.as_raw_socket();
+        assert!(socket != winapi::um::winsock2::INVALID_SOCKET as _);
         SockRef {
-            socket: ManuallyDrop::new(unsafe { Socket::from_raw_socket(socket.as_raw_socket()) }),
+            socket: ManuallyDrop::new(unsafe { Socket::from_raw_socket(socket) }),
             _lifetime: PhantomData,
         }
     }