diff --git a/warp/Network/Wai/Handler/Warp/Run.hs b/warp/Network/Wai/Handler/Warp/Run.hs
index 858d893a2..0cde00d90 100644
--- a/warp/Network/Wai/Handler/Warp/Run.hs
+++ b/warp/Network/Wai/Handler/Warp/Run.hs
@@ -386,8 +386,8 @@ serveConnection conn ii th origAddr transport settings app = do
         if isHTTP2 transport
             then return (True, "")
             else do
-                bs0 <- connRecv conn
-                if S.length bs0 >= 4 && "PRI " `S.isPrefixOf` bs0
+                bs0 <- recv4 ""
+                if "PRI " `S.isPrefixOf` bs0
                     then return (True, bs0)
                     else return (False, bs0)
     if settingsHTTP2Enabled settings && h2
@@ -395,6 +395,18 @@ serveConnection conn ii th origAddr transport settings app = do
             http2 settings ii conn transport app origAddr th bs
         else do
             http1 settings ii conn transport app origAddr th bs
+  where
+    recv4 bs0 = do
+        bs1 <- connRecv conn
+        if S.null bs1 then
+            return bs0
+          else do
+            -- In the case where bs0 is "", (<>) is called unnecessarily.
+            -- But we adopt this logic for simplicity.
+            let bs2 = bs0 <> bs1
+            if S.length bs2 >= 4
+                 then return bs2
+                 else recv4 bs2
 
 -- | Set flag FileCloseOnExec flag on a socket (on Unix)
 --