diff --git a/include/type.hpp b/include/type.hpp
index 85d0ff7d..7ce29151 100644
--- a/include/type.hpp
+++ b/include/type.hpp
@@ -190,7 +190,9 @@ class RecordType : public Type {
       const noexcept = 0;
   /// @return The type offset in the record.
   virtual std::size_t offset(  // NOLINT(readability-identifier-naming)
-      const std::string& id) const noexcept = 0;
+      const std::string& id) const = 0;
+  virtual std::size_t offset(  // NOLINT(readability-identifier-naming)
+      const std::size_t id) const = 0;
   /// @brief Checks if `id` is a member of the record type.
   virtual bool IsMember(const std::string& id) const noexcept = 0;
   /// @return The type of a member in struct or union. The unknown type if the
@@ -209,7 +211,9 @@ class StructType : public RecordType {
   std::string id()  // NOLINT(readability-identifier-naming)
       const noexcept override;
   std::size_t offset(  // NOLINT(readability-identifier-naming)
-      const std::string& id) const noexcept override;
+      const std::string& id) const override;
+  std::size_t offset(  // NOLINT(readability-identifier-naming)
+      const std::size_t index) const override;
   bool IsMember(const std::string& id) const noexcept override;
   std::unique_ptr<Type> MemberType(
       const std::string& id) const noexcept override;
@@ -238,7 +242,9 @@ class UnionType : public RecordType {
   std::string id()  // NOLINT(readability-identifier-naming)
       const noexcept override;
   std::size_t offset(  // NOLINT(readability-identifier-naming)
-      const std::string& id) const noexcept override;
+      const std::string& id) const override;
+  std::size_t offset(  // NOLINT(readability-identifier-naming)
+      const std::size_t index) const override;
   bool IsMember(const std::string& id) const noexcept override;
   std::unique_ptr<Type> MemberType(
       const std::string& id) const noexcept override;
diff --git a/src/qbe_ir_generator.cpp b/src/qbe_ir_generator.cpp
index 2e51028d..ff162f17 100644
--- a/src/qbe_ir_generator.cpp
+++ b/src/qbe_ir_generator.cpp
@@ -225,10 +225,18 @@ void QbeIrGenerator::Visit(const RecordVarDeclNode& record_var_decl) {
     init->Accept(*this);
     const auto init_num = num_recorder.NumOfPrevExpr();
 
+    // NOTE: Every member shares the same starting memory location in union.
+    // Abort if initializing more than one element.
+    if (record_var_decl.type->IsUnion() && i > 0) {
+      break;
+    }
+
     // res_addr = base_addr + offset
     const int res_addr_num = NextLocalNum();
+    auto* record_type = dynamic_cast<RecordType*>(record_var_decl.type.get());
+    assert(record_type);
     WriteInstr_("{} =l add {}, {}", FuncScopeTemp{res_addr_num},
-                FuncScopeTemp{base_addr}, i * init->type->size());
+                FuncScopeTemp{base_addr}, record_type->offset(i));
     WriteInstr_("storew {}, {}", FuncScopeTemp{init_num},
                 FuncScopeTemp{res_addr_num});
   }
diff --git a/src/type.cpp b/src/type.cpp
index 426b7018..cff629a9 100644
--- a/src/type.cpp
+++ b/src/type.cpp
@@ -158,15 +158,32 @@ std::string StructType::id() const noexcept {
   return id_;
 }
 
-std::size_t StructType::offset(const std::string& id) const noexcept {
+std::size_t StructType::offset(const std::string& id) const {
+  std::size_t offset = 0;
   for (auto i = std::size_t{0}, e = fields_.size(); i < e; ++i) {
     const auto& field = fields_.at(i);
     if (field->id == id) {
-      return i * field->type->size();
+      return offset;
     }
+
+    offset += field->type->size();
   }
 
-  return -1;
+  throw "member not found in struct!";
+}
+
+std::size_t StructType::offset(const std::size_t index) const {
+  std::size_t offset = 0;
+  for (auto i = std::size_t{0}, e = fields_.size(); i < e; ++i) {
+    const auto& field = fields_.at(i);
+    if (i == index) {
+      return offset;
+    }
+
+    offset += field->type->size();
+  }
+
+  throw "member not found in struct!";
 }
 
 bool StructType::IsMember(const std::string& id) const noexcept {
@@ -231,7 +248,12 @@ std::unique_ptr<Type> StructType::Clone() const {
   return std::make_unique<StructType>(id_, std::move(cloned_fields));
 }
 
-std::size_t UnionType::offset(const std::string& id) const noexcept {
+std::size_t UnionType::offset(const std::string& id) const {
+  // Every member in union shares the same starting location.
+  return 0;
+}
+
+std::size_t UnionType::offset(const std::size_t index) const {
   // Every member in union shares the same starting location.
   return 0;
 }