diff --git a/quic/core/quic_stream_sequencer_buffer.cc b/quic/core/quic_stream_sequencer_buffer.cc
index f4ff276..e68925b 100644
--- a/quic/core/quic_stream_sequencer_buffer.cc
+++ b/quic/core/quic_stream_sequencer_buffer.cc
@@ -500,12 +500,12 @@
   }
 }
 
-std::string QuicStreamSequencerBuffer::GapsDebugString() {
+std::string QuicStreamSequencerBuffer::GapsDebugString() const {
   // TODO(vasilvv): this should return the complement of |bytes_received_|.
   return bytes_received_.ToString();
 }
 
-std::string QuicStreamSequencerBuffer::ReceivedFramesDebugString() {
+std::string QuicStreamSequencerBuffer::ReceivedFramesDebugString() const {
   return bytes_received_.ToString();
 }
 
diff --git a/quic/core/quic_stream_sequencer_buffer.h b/quic/core/quic_stream_sequencer_buffer.h
index 6347b1c..b0478c0 100644
--- a/quic/core/quic_stream_sequencer_buffer.h
+++ b/quic/core/quic_stream_sequencer_buffer.h
@@ -219,10 +219,10 @@
   QuicStreamOffset NextExpectedByte() const;
 
   // Return |gaps_| as a string: [1024, 1500) [1800, 2048)... for debugging.
-  std::string GapsDebugString();
+  std::string GapsDebugString() const;
 
   // Return all received frames as a string in same format as GapsDebugString();
-  std::string ReceivedFramesDebugString();
+  std::string ReceivedFramesDebugString() const;
 
   // The maximum total capacity of this buffer in byte, as constructed.
   const size_t max_buffer_capacity_bytes_;