diff --git a/React/Base/RCTJavaScriptLoader.mm b/React/Base/RCTJavaScriptLoader.mm
index 60fa2c68212..9258cea737e 100755
--- a/React/Base/RCTJavaScriptLoader.mm
+++ b/React/Base/RCTJavaScriptLoader.mm
@@ -114,7 +114,7 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
     return nil;
   }
 
-  facebook::react::BundleHeader header{};
+  facebook::react::BundleHeader header;
   size_t readResult = fread(&header, sizeof(header), 1, bundle);
   fclose(bundle);
   if (readResult != 1) {
@@ -151,11 +151,11 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
       }
       return nil;
     }
-    else if ((uint32_t)runtimeBCVersion != header.BCVersion) {
+    else if ((uint32_t)runtimeBCVersion != header.version) {
       if (error) {
         NSString *errDesc =
           [NSString stringWithFormat:@"BC Version Mismatch. Expect: %d, Actual: %u",
-                    runtimeBCVersion, header.BCVersion];
+                    runtimeBCVersion, header.version];
 
         *error = [NSError errorWithDomain:RCTJavaScriptLoaderErrorDomain
                                      code:RCTJavaScriptLoaderErrorBCVersion
diff --git a/React/CxxBridge/RCTCxxBridge.mm b/React/CxxBridge/RCTCxxBridge.mm
index c8d19c14ed9..8964d161865 100644
--- a/React/CxxBridge/RCTCxxBridge.mm
+++ b/React/CxxBridge/RCTCxxBridge.mm
@@ -87,7 +87,7 @@ static JSValueRef nativePerformanceNow(
 }
 
 static bool isRAMBundle(NSData *script) {
-  BundleHeader header{};
+  BundleHeader header;
   [script getBytes:&header length:sizeof(header)];
   return parseTypeFromHeader(header) == ScriptTag::RAMBundle;
 }
diff --git a/React/Executors/RCTJSCExecutor.mm b/React/Executors/RCTJSCExecutor.mm
index 92918feb602..3b5a1816fef 100644
--- a/React/Executors/RCTJSCExecutor.mm
+++ b/React/Executors/RCTJSCExecutor.mm
@@ -706,7 +706,7 @@ static TaggedScript loadTaggedScript(NSData *script,
 {
   RCT_PROFILE_BEGIN_EVENT(0, @"executeApplicationScript / prepare bundle", nil);
 
-  facebook::react::BundleHeader header{};
+  facebook::react::BundleHeader header;
   [script getBytes:&header length:sizeof(header)];
   facebook::react::ScriptTag tag = facebook::react::parseTypeFromHeader(header);
 
diff --git a/ReactAndroid/src/main/jni/xreact/jni/CatalystInstanceImpl.cpp b/ReactAndroid/src/main/jni/xreact/jni/CatalystInstanceImpl.cpp
index 77cd311d350..b4aa91b631c 100644
--- a/ReactAndroid/src/main/jni/xreact/jni/CatalystInstanceImpl.cpp
+++ b/ReactAndroid/src/main/jni/xreact/jni/CatalystInstanceImpl.cpp
@@ -194,7 +194,7 @@ bool CatalystInstanceImpl::isIndexedRAMBundle(const char *sourcePath) {
   if (!bundle_stream) {
     return false;
   }
-  BundleHeader header{};
+  BundleHeader header;
   bundle_stream.read(reinterpret_cast<char *>(&header), sizeof(header));
   bundle_stream.close();
   return parseTypeFromHeader(header) == ScriptTag::RAMBundle;
diff --git a/ReactCommon/cxxreact/JSBundleType.cpp b/ReactCommon/cxxreact/JSBundleType.cpp
index a53e0097743..071abd7f389 100644
--- a/ReactCommon/cxxreact/JSBundleType.cpp
+++ b/ReactCommon/cxxreact/JSBundleType.cpp
@@ -7,18 +7,18 @@ namespace facebook {
 namespace react {
 
 static uint32_t constexpr RAMBundleMagicNumber = 0xFB0BD1E5;
-static uint64_t constexpr BCBundleMagicNumber  = 0xFF4865726D657300;
+static uint32_t constexpr BCBundleMagicNumber  = 0x6D657300;
 
 ScriptTag parseTypeFromHeader(const BundleHeader& header) {
-  if (littleEndianToHost(header.RAMMagic) == RAMBundleMagicNumber) {
+
+  switch (littleEndianToHost(header.magic)) {
+  case RAMBundleMagicNumber:
     return ScriptTag::RAMBundle;
-  }
-
-  if (littleEndianToHost(header.BCMagic) == BCBundleMagicNumber) {
+  case BCBundleMagicNumber:
     return ScriptTag::BCBundle;
+  default:
+    return ScriptTag::String;
   }
-
-  return ScriptTag::String;
 }
 
 const char *stringForScriptTag(const ScriptTag& tag) {
diff --git a/ReactCommon/cxxreact/JSBundleType.h b/ReactCommon/cxxreact/JSBundleType.h
index 1945fa4c779..3b89ecb2b7a 100644
--- a/ReactCommon/cxxreact/JSBundleType.h
+++ b/ReactCommon/cxxreact/JSBundleType.h
@@ -27,16 +27,14 @@ enum struct ScriptTag {
  * 4 bytes, for BC bundles this is 12 bytes. This structure holds the first 12
  * bytes from a bundle in a way that gives access to that information.
  */
-union BundleHeader {
+struct __attribute__((packed)) BundleHeader {
   BundleHeader() {
     std::memset(this, 0, sizeof(BundleHeader));
   }
 
-  uint32_t RAMMagic;
-  struct {
-    uint64_t BCMagic;
-    uint32_t BCVersion;
-  };
+  uint32_t magic;
+  uint32_t reserved_;
+  uint32_t version;
 };
 
 /**
diff --git a/ReactCommon/cxxreact/JSCExecutor.cpp b/ReactCommon/cxxreact/JSCExecutor.cpp
index 4dd5139b392..bfc35e76f5b 100644
--- a/ReactCommon/cxxreact/JSCExecutor.cpp
+++ b/ReactCommon/cxxreact/JSCExecutor.cpp
@@ -386,7 +386,7 @@ void JSCExecutor::loadApplicationScript(std::unique_ptr<const JSBigString> scrip
     }
   }
 #elif defined(__APPLE__)
-  BundleHeader header{};
+  BundleHeader header;
   memcpy(&header, script->c_str(), std::min(script->size(), sizeof(BundleHeader)));
   auto scriptTag = parseTypeFromHeader(header);