diff --git a/internal/flowcontrol/base_flow_controller.go b/internal/flowcontrol/base_flow_controller.go
index 46825c424..eaa971e5a 100644
--- a/internal/flowcontrol/base_flow_controller.go
+++ b/internal/flowcontrol/base_flow_controller.go
@@ -15,7 +15,7 @@ type baseFlowController struct {
 	lastBlockedAt protocol.ByteCount
 
 	// for receiving data
-	mutex                sync.RWMutex
+	mutex                sync.Mutex
 	bytesRead            protocol.ByteCount
 	highestReceived      protocol.ByteCount
 	receiveWindow        protocol.ByteCount