diff --git a/src/net/http/httptest/server.go b/src/net/http/httptest/server.go
index 4f85ff55d8..1c0c0f6987 100644
--- a/src/net/http/httptest/server.go
+++ b/src/net/http/httptest/server.go
@@ -317,21 +317,17 @@ func (s *Server) wrap() {
 		s.mu.Lock()
 		defer s.mu.Unlock()
 
-		// Keep Close from returning until the user's ConnState hook
-		// (if any) finishes. Without this, the call to forgetConn
-		// below might send the count to 0 before we run the hook.
-		s.wg.Add(1)
-		defer s.wg.Done()
-
 		switch cs {
 		case http.StateNew:
-			s.wg.Add(1)
 			if _, exists := s.conns[c]; exists {
 				panic("invalid state transition")
 			}
 			if s.conns == nil {
 				s.conns = make(map[net.Conn]http.ConnState)
 			}
+			// Add c to the set of tracked conns and increment it to the
+			// waitgroup.
+			s.wg.Add(1)
 			s.conns[c] = cs
 			if s.closed {
 				// Probably just a socket-late-binding dial from
@@ -358,7 +354,14 @@ func (s *Server) wrap() {
 				s.closeConn(c)
 			}
 		case http.StateHijacked, http.StateClosed:
-			s.forgetConn(c)
+			// Remove c from the set of tracked conns and decrement it from the
+			// waitgroup, unless it was previously removed.
+			if _, ok := s.conns[c]; ok {
+				delete(s.conns, c)
+				// Keep Close from returning until the user's ConnState hook
+				// (if any) finishes.
+				defer s.wg.Done()
+			}
 		}
 		if oldHook != nil {
 			oldHook(c, cs)
@@ -378,13 +381,3 @@ func (s *Server) closeConnChan(c net.Conn, done chan<- struct{}) {
 		done <- struct{}{}
 	}
 }
-
-// forgetConn removes c from the set of tracked conns and decrements it from the
-// waitgroup, unless it was previously removed.
-// s.mu must be held.
-func (s *Server) forgetConn(c net.Conn) {
-	if _, ok := s.conns[c]; ok {
-		delete(s.conns, c)
-		s.wg.Done()
-	}
-}
diff --git a/src/net/http/httptest/server_test.go b/src/net/http/httptest/server_test.go
index 39568b358c..5313f65456 100644
--- a/src/net/http/httptest/server_test.go
+++ b/src/net/http/httptest/server_test.go
@@ -9,6 +9,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"sync"
 	"testing"
 )
 
@@ -203,6 +204,59 @@ func TestServerZeroValueClose(t *testing.T) {
 	ts.Close() // tests that it doesn't panic
 }
 
+// Issue 51799: test hijacking a connection and then closing it
+// concurrently with closing the server.
+func TestCloseHijackedConnection(t *testing.T) {
+	hijacked := make(chan net.Conn)
+	ts := NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		defer close(hijacked)
+		hj, ok := w.(http.Hijacker)
+		if !ok {
+			t.Fatal("failed to hijack")
+		}
+		c, _, err := hj.Hijack()
+		if err != nil {
+			t.Fatal(err)
+		}
+		hijacked <- c
+	}))
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		req, err := http.NewRequest("GET", ts.URL, nil)
+		if err != nil {
+			t.Log(err)
+		}
+		// Use a client not associated with the Server.
+		var c http.Client
+		resp, err := c.Do(req)
+		if err != nil {
+			t.Log(err)
+			return
+		}
+		resp.Body.Close()
+	}()
+
+	wg.Add(1)
+	conn := <-hijacked
+	go func(conn net.Conn) {
+		defer wg.Done()
+		// Close the connection and then inform the Server that
+		// we closed it.
+		conn.Close()
+		ts.Config.ConnState(conn, http.StateClosed)
+	}(conn)
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		ts.Close()
+	}()
+	wg.Wait()
+}
+
 func TestTLSServerWithHTTP2(t *testing.T) {
 	modes := []struct {
 		name      string