diff --git a/src/coreclr/jit/valuenum.cpp b/src/coreclr/jit/valuenum.cpp
index 2ce97d285206cb..1d13229cf96a19 100644
--- a/src/coreclr/jit/valuenum.cpp
+++ b/src/coreclr/jit/valuenum.cpp
@@ -10299,7 +10299,9 @@ void ValueNumStore::vnDumpSimdType(Compiler* comp, VNFuncApp* simdType)
     int         simdSize    = ConstantValue<int>(simdType->m_args[0]);
     CorInfoType baseJitType = (CorInfoType)ConstantValue<int>(simdType->m_args[1]);
 
-    printf("%s(simd%d, %s)", VNFuncName(simdType->m_func), simdSize, varTypeName(JitType2PreciseVarType(baseJitType)));
+    printf("%s(simd%d, %s)", VNFuncName(simdType->m_func), simdSize,
+           baseJitType == CORINFO_TYPE_UNDEF ? varTypeName(TYP_UNDEF)
+                                             : varTypeName(JitType2PreciseVarType(baseJitType)));
 }
 #endif // FEATURE_SIMD
 
diff --git a/src/coreclr/vm/corelib.h b/src/coreclr/vm/corelib.h
index 4cffd124370fec..38a05866ff3e2c 100644
--- a/src/coreclr/vm/corelib.h
+++ b/src/coreclr/vm/corelib.h
@@ -1104,6 +1104,7 @@ DEFINE_FIELD_U(_condition,          ContractExceptionObject,    _Condition)
 DEFINE_CLASS(MODULEBASE,        Reflection,         Module)
 
 DEFINE_CLASS(STACKALLOCATEDBOX,   CompilerServices,     StackAllocatedBox`1)
+DEFINE_FIELD(STACKALLOCATEDBOX,   VALUE,                _value)
 
 DEFINE_CLASS(UTF8STRINGMARSHALLER, Marshalling, Utf8StringMarshaller)
 DEFINE_METHOD(UTF8STRINGMARSHALLER, CONVERT_TO_MANAGED, ConvertToManaged, SM_PtrByte_RetStr)
diff --git a/src/coreclr/vm/jitinterface.cpp b/src/coreclr/vm/jitinterface.cpp
index e7572863016f08..2c470710f0b4c2 100644
--- a/src/coreclr/vm/jitinterface.cpp
+++ b/src/coreclr/vm/jitinterface.cpp
@@ -6168,6 +6168,12 @@ CORINFO_CLASS_HANDLE  CEEInfo::getTypeForBoxOnStack(CORINFO_CLASS_HANDLE cls)
         TypeHandle stackAllocatedBox = CoreLibBinder::GetClass(CLASS__STACKALLOCATEDBOX);
         TypeHandle stackAllocatedBoxInst = stackAllocatedBox.Instantiate(boxedFieldsInst);
         result = static_cast<CORINFO_CLASS_HANDLE>(stackAllocatedBoxInst.AsPtr());
+
+#ifdef _DEBUG
+        FieldDesc* pValueFD = CoreLibBinder::GetField(FIELD__STACKALLOCATEDBOX__VALUE);
+        DWORD index = pValueFD->GetApproxEnclosingMethodTable()->GetIndexForFieldDesc(pValueFD);
+        _ASSERTE(stackAllocatedBoxInst.GetMethodTable()->GetFieldDescByIndex(index)->GetOffset() == TARGET_POINTER_SIZE);
+#endif
     }
 
     EE_TO_JIT_TRANSITION();
diff --git a/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/StackAllocatedBox.cs b/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/StackAllocatedBox.cs
index 868edb18c1594b..33f2131edd115a 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/StackAllocatedBox.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/StackAllocatedBox.cs
@@ -7,7 +7,7 @@
 namespace System.Runtime.CompilerServices
 {
     [NonVersionable]
-    [StructLayout(LayoutKind.Sequential)]
+    [StructLayout(LayoutKind.Sequential, Pack = 1)]
     internal unsafe struct StackAllocatedBox<T>
     {
         // These fields are only accessed from jitted code