diff --git a/modules/video_coding/frame_buffer2.cc b/modules/video_coding/frame_buffer2.cc
index f314ccb..5b81c5f 100644
--- a/modules/video_coding/frame_buffer2.cc
+++ b/modules/video_coding/frame_buffer2.cc
@@ -248,9 +248,13 @@
 }
 
 bool FrameBuffer::ValidReferences(const FrameObject& frame) const {
+  if (frame.picture_id < 0)
+    return false;
+
   for (size_t i = 0; i < frame.num_references; ++i) {
-    if (AheadOrAt<uint16_t>(frame.references[i], frame.picture_id))
+    if (frame.references[i] < 0 || frame.references[i] >= frame.picture_id)
       return false;
+
     for (size_t j = i + 1; j < frame.num_references; ++j) {
       if (frame.references[i] == frame.references[j])
         return false;
diff --git a/modules/video_coding/frame_buffer2.h b/modules/video_coding/frame_buffer2.h
index 5f5173e..77fe485 100644
--- a/modules/video_coding/frame_buffer2.h
+++ b/modules/video_coding/frame_buffer2.h
@@ -76,19 +76,19 @@
 
  private:
   struct FrameKey {
-    FrameKey() : picture_id(0), spatial_layer(0) {}
-    FrameKey(uint16_t picture_id, uint8_t spatial_layer)
+    FrameKey() : picture_id(-1), spatial_layer(0) {}
+    FrameKey(int64_t picture_id, uint8_t spatial_layer)
         : picture_id(picture_id), spatial_layer(spatial_layer) {}
 
     bool operator<(const FrameKey& rhs) const {
       if (picture_id == rhs.picture_id)
         return spatial_layer < rhs.spatial_layer;
-      return AheadOf(rhs.picture_id, picture_id);
+      return picture_id < rhs.picture_id;
     }
 
     bool operator<=(const FrameKey& rhs) const { return !(rhs < *this); }
 
-    uint16_t picture_id;
+    int64_t picture_id;
     uint8_t spatial_layer;
   };
 
diff --git a/modules/video_coding/rtp_frame_reference_finder_unittest.cc b/modules/video_coding/rtp_frame_reference_finder_unittest.cc
index 5500430..a40a727 100644
--- a/modules/video_coding/rtp_frame_reference_finder_unittest.cc
+++ b/modules/video_coding/rtp_frame_reference_finder_unittest.cc
@@ -54,7 +54,7 @@
 class TestRtpFrameReferenceFinder : public ::testing::Test,
                                     public OnCompleteFrameCallback {
  protected:
-  static constexpr uint64_t kUnwrappedSequenceStart = 10000000000000000000UL;
+  static constexpr uint64_t kUnwrappedSequenceStart = 1000000000000000000UL;
 
   TestRtpFrameReferenceFinder()
       : rand_(0x8739211),
diff --git a/modules/video_coding/sequence_number_util.h b/modules/video_coding/sequence_number_util.h
index 3e7d64a..053f9d0 100644
--- a/modules/video_coding/sequence_number_util.h
+++ b/modules/video_coding/sequence_number_util.h
@@ -91,11 +91,11 @@
       "Type unwrapped must be an unsigned integer smaller than uint64_t.");
 
  public:
-  // We want a value that is close to 2^63 for two reasons. Firstly, we
-  // can unwrap wrapping numbers in either direction, and secondly, we avoid
-  // causing a crash on bad input. We also want the default value to be somewhat
-  // human readable, a power of 10 for example.
-  static constexpr uint64_t kDefaultStartValue = 10000000000000000000UL;
+  // We want a default value that is close to 2^62 for a two reasons. Firstly,
+  // we can unwrap wrapping numbers in either direction, and secondly, the
+  // unwrapped numbers can be stored in either int64_t or uint64_t. We also want
+  // the default value to be human readable, which makes a power of 10 suitable.
+  static constexpr uint64_t kDefaultStartValue = 1000000000000000000UL;
 
   SeqNumUnwrapper() : last_unwrapped_(kDefaultStartValue) {}
   explicit SeqNumUnwrapper(uint64_t start_at) : last_unwrapped_(start_at) {}