diff --git a/src/CommonFileFormats.hh b/src/CommonFileFormats.hh
index 60e8cfe6..d039025a 100644
--- a/src/CommonFileFormats.hh
+++ b/src/CommonFileFormats.hh
@@ -1,7 +1,9 @@
 #pragma once
 
 #include <phosg/Encoding.hh>
+#include <phosg/Strings.hh>
 #include <phosg/Vector.hh>
+#include <set>
 
 #include "Text.hh"
 
@@ -160,3 +162,23 @@ struct RELFileFooterT {
 } __packed_ws_be__(RELFileFooterT, 0x20);
 using RELFileFooter = RELFileFooterT<false>;
 using RELFileFooterBE = RELFileFooterT<true>;
+
+template <bool BE>
+std::set<uint32_t> all_relocation_offsets_for_rel_file(const void* data, size_t size) {
+  phosg::StringReader r(data, size);
+
+  std::set<uint32_t> ret;
+  ret.emplace(r.size() - 0x20); // REL footer
+  ret.emplace(r.pget<U32T<BE>>(r.size() - 0x10)); // root
+  ret.emplace(r.pget<U32T<BE>>(r.size() - 0x20)); // relocations
+
+  const auto& footer = r.pget<RELFileFooterT<BE>>(r.size() - sizeof(RELFileFooterT<BE>));
+  auto sub_r = r.sub(footer.relocations_offset, footer.num_relocations * sizeof(U16T<BE>));
+  uint32_t offset = 0;
+  while (!sub_r.eof()) {
+    offset += sub_r.template get<U16T<BE>>() * 4;
+    ret.emplace(r.pget<U32T<BE>>(offset));
+  }
+
+  return ret;
+}
diff --git a/src/ItemParameterTable.cc b/src/ItemParameterTable.cc
index 8788762f..666a2a32 100644
--- a/src/ItemParameterTable.cc
+++ b/src/ItemParameterTable.cc
@@ -2902,21 +2902,10 @@ public:
   }
 
   const std::set<uint32_t>& all_start_offsets() const {
-    if (!this->start_offsets.has_value()) {
-      auto& ret = this->start_offsets.emplace();
-      ret.emplace(r.size() - 0x20); // REL footer
-      ret.emplace(BE ? r.pget_u32b(r.size() - 0x10) : r.pget_u32l(r.size() - 0x10)); // root
-      ret.emplace(BE ? r.pget_u32b(r.size() - 0x20) : r.pget_u32l(r.size() - 0x20)); // relocations
-
-      const auto& footer = r.pget<RELFileFooterT<BE>>(r.size() - sizeof(RELFileFooterT<BE>));
-      auto sub_r = r.sub(footer.relocations_offset, footer.num_relocations * sizeof(U16T<BE>));
-      uint32_t offset = 0;
-      while (!sub_r.eof()) {
-        offset += sub_r.template get<U16T<BE>>() * 4;
-        ret.emplace(r.pget<U32T<BE>>(offset));
-      }
+    if (this->start_offsets.empty()) {
+      this->start_offsets = all_relocation_offsets_for_rel_file<BE>(r.pgetv(0, r.size()), r.size());
     }
-    return *this->start_offsets;
+    return this->start_offsets;
   }
 
   size_t get_data_range_size(size_t start_offset) const {
@@ -3270,7 +3259,7 @@ protected:
   phosg::StringReader r;
   const RootT* root;
 
-  mutable std::optional<std::set<uint32_t>> start_offsets;
+  mutable std::set<uint32_t> start_offsets;
 
   mutable std::unordered_map<uint16_t, Weapon> weapons;
   mutable std::vector<ArmorOrShield> armors;