--- src/objects/js-temporal-zoneinfo64.cc.orig	2026-10-08 06:17:53 UTC
+++ src/objects/js-temporal-zoneinfo64.cc
@@ -11,7 +11,11 @@
 #include "temporal_rs/TimeZone.hpp"
 
 #ifdef V8_INTL_SUPPORT
-#include "udatamem.h"
+#include "unicode/udata.h"
+// udatamem.h is internal to ICU and not installed by devel/icu; declare the
+// two accessors from it that libicuuc exports.
+U_CAPI int32_t U_EXPORT2 udata_getLength(const UDataMemory* pData);
+U_CAPI const void* U_EXPORT2 udata_getRawMemory(const UDataMemory* pData);
 #else
 // Defined in builtins-temporal-zoneinfo64-data.cc, generated by
 // include-file-as-bytes.py
@@ -32,8 +36,16 @@ ZoneInfo64Provider::ZoneInfo64Provider() {
   }
   // NOT udata_getLength: this ignores the header,
   // and we're parsing resb files with the header
-  auto length = memory->length;
+  // Without UDataMemory's definition, add the header back to what
+  // udata_getLength reports.
   const void* data = udata_getRawMemory(memory);
+  auto body_length = udata_getLength(memory);
+  if (body_length < 0) {
+    provider = temporal_rs::Provider::empty();
+    return;
+  }
+  auto length = body_length + (static_cast<const char*>(udata_getMemory(memory)) -
+                               static_cast<const char*>(data));
   DCHECK_WITH_MSG(length % 4 == 0, "ICU4C should align udata to uint32_t");
   if (length % 4 != 0) {
     // This really shouldn't happen: ICU4C aligns these files