diff --git a/src/Compression.cc b/src/Compression.cc
index 36328358..541bc8a9 100644
--- a/src/Compression.cc
+++ b/src/Compression.cc
@@ -394,7 +394,7 @@ string bc0_compress(
 
   parray<uint8_t, 0x1000> memo;
   uint16_t memo_offset = 0x0FEE;
-  vector<unordered_set<size_t>> memo_index(0x100);
+  vector<set<size_t>> memo_index(0x100);
   auto write_memo = [&](uint8_t new_v) -> void {
     uint8_t existing_v = memo[memo_offset];
     if (existing_v != new_v) {
diff --git a/src/Compression.hh b/src/Compression.hh
index 72669de3..03119658 100644
--- a/src/Compression.hh
+++ b/src/Compression.hh
@@ -4,6 +4,7 @@
 
 #include <string>
 #include <functional>
+#include <set>
 
 #include "Text.hh"
 
@@ -50,7 +51,7 @@ private:
   parray<uint8_t, 0x100> forward_log;
   size_t compression_offset;
   parray<uint8_t, 0x2000> reverse_log;
-  std::vector<std::unordered_set<size_t>> reverse_log_index;
+  std::vector<std::set<size_t>> reverse_log_index;
 
   StringWriter output;
 };