diff --git a/lib/fizzy/execute.cpp b/lib/fizzy/execute.cpp
index ee77f94597..ff54229560 100644
--- a/lib/fizzy/execute.cpp
+++ b/lib/fizzy/execute.cpp
@@ -453,7 +453,7 @@ __attribute__((no_sanitize("float-cast-overflow"))) inline constexpr float demot
     return static_cast<float>(value);
 }
 
-void branch(const Code& code, OperandStack& stack, const Instr*& pc, const uint8_t*& immediates,
+void branch(const Code& code, OperandStack& stack, const uint8_t*& pc, const uint8_t*& immediates,
     uint32_t arity) noexcept
 {
     const auto code_offset = read<uint32_t>(immediates);
@@ -531,12 +531,12 @@ ExecutionResult execute(Instance& instance, FuncIdx func_idx, const Value* args,
     OperandStack stack(args, func_type.inputs.size(), code.local_count,
         static_cast<size_t>(code.max_stack_height));
 
-    const Instr* pc = code.instructions.data();
+    const uint8_t* pc = code.instructions.data();
     const uint8_t* immediates = code.immediates.data();
 
     while (true)
     {
-        const auto instruction = *pc++;
+        const auto instruction = static_cast<Instr>(*pc++);
         switch (instruction)
         {
         case Instr::unreachable:
diff --git a/lib/fizzy/parser_expr.cpp b/lib/fizzy/parser_expr.cpp
index 28bc47a00f..f0ced862dc 100644
--- a/lib/fizzy/parser_expr.cpp
+++ b/lib/fizzy/parser_expr.cpp
@@ -869,7 +869,7 @@ parser_result<Code> parse_expr(const uint8_t* pos, const uint8_t* end, FuncIdx f
             break;
         }
         }
-        code.instructions.emplace_back(instr);
+        code.instructions.emplace_back(opcode);
     }
     assert(control_stack.empty());
     return {code, pos};
diff --git a/lib/fizzy/types.hpp b/lib/fizzy/types.hpp
index 6fccae4e29..06fbbd2391 100644
--- a/lib/fizzy/types.hpp
+++ b/lib/fizzy/types.hpp
@@ -362,7 +362,7 @@ struct Code
 
     // The instructions bytecode without immediate values.
     // https://webassembly.github.io/spec/core/binary/instructions.html
-    std::vector<Instr> instructions;
+    std::vector<uint8_t> instructions;
 
     // The decoded instructions' immediate values.
     // These are instruction-type dependent fixed size value in the order of instructions.
diff --git a/test/unittests/execute_death_test.cpp b/test/unittests/execute_death_test.cpp
index b94610a5cd..51539671bd 100644
--- a/test/unittests/execute_death_test.cpp
+++ b/test/unittests/execute_death_test.cpp
@@ -11,11 +11,11 @@ using namespace fizzy;
 #if !defined(NDEBUG) || __has_feature(undefined_behavior_sanitizer)
 TEST(execute_death, malformed_instruction_opcode)
 {
-    constexpr auto malformed_opcode = static_cast<Instr>(6);
+    constexpr uint8_t malformed_opcode = 6;
 
     Code code;
     code.instructions.emplace_back(malformed_opcode);
-    code.instructions.emplace_back(Instr::end);
+    code.instructions.emplace_back(static_cast<uint8_t>(Instr::end));
 
     auto module = std::make_unique<Module>();
     module->typesec.emplace_back(FuncType{});
diff --git a/test/unittests/execute_numeric_test.cpp b/test/unittests/execute_numeric_test.cpp
index f56671686e..4662d2dc15 100644
--- a/test/unittests/execute_numeric_test.cpp
+++ b/test/unittests/execute_numeric_test.cpp
@@ -20,7 +20,10 @@ ExecutionResult execute_unary_operation(Instr instr, uint64_t arg)
     // type is currently needed only to get arity of function, so exact value types don't matter
     module->typesec.emplace_back(FuncType{{ValType::i32}, {ValType::i32}});
     module->funcsec.emplace_back(TypeIdx{0});
-    module->codesec.emplace_back(Code{1, 0, {Instr::local_get, instr, Instr::end}, {0, 0, 0, 0}});
+    module->codesec.emplace_back(Code{1, 0,
+        {static_cast<uint8_t>(Instr::local_get), static_cast<uint8_t>(instr),
+            static_cast<uint8_t>(Instr::end)},
+        {0, 0, 0, 0}});
 
     return execute(*instantiate(std::move(module)), 0, {arg});
 }
@@ -31,8 +34,10 @@ ExecutionResult execute_binary_operation(Instr instr, uint64_t lhs, uint64_t rhs
     // type is currently needed only to get arity of function, so exact value types don't matter
     module->typesec.emplace_back(FuncType{{ValType::i32, ValType::i32}, {ValType::i32}});
     module->funcsec.emplace_back(TypeIdx{0});
-    module->codesec.emplace_back(Code{
-        2, 0, {Instr::local_get, Instr::local_get, instr, Instr::end}, {0, 0, 0, 0, 1, 0, 0, 0}});
+    module->codesec.emplace_back(Code{2, 0,
+        {static_cast<uint8_t>(Instr::local_get), static_cast<uint8_t>(Instr::local_get),
+            static_cast<uint8_t>(instr), static_cast<uint8_t>(Instr::end)},
+        {0, 0, 0, 0, 1, 0, 0, 0}});
 
     return execute(*instantiate(std::move(module)), 0, {lhs, rhs});
 }
diff --git a/test/unittests/execute_test.cpp b/test/unittests/execute_test.cpp
index d7b4032ff4..8611afe4a9 100644
--- a/test/unittests/execute_test.cpp
+++ b/test/unittests/execute_test.cpp
@@ -380,7 +380,7 @@ TEST(execute, i32_load_all_variants)
         from_hex("0061736d0100000001060160017f017f030201000504010101010a0901070020002802000b");
     const auto module = parse(wasm);
 
-    auto& load_instr = const_cast<Instr&>(module->codesec[0].instructions[1]);
+    auto& load_instr = const_cast<uint8_t&>(module->codesec[0].instructions[1]);
     ASSERT_EQ(load_instr, Instr::i32_load);
     ASSERT_EQ(module->codesec[0].immediates.substr(4), "00000000"_bytes);  // load offset.
 
@@ -396,7 +396,7 @@ TEST(execute, i32_load_all_variants)
 
     for (const auto& test_case : test_cases)
     {
-        load_instr = std::get<0>(test_case);
+        load_instr = static_cast<uint8_t>(std::get<0>(test_case));
         auto instance = instantiate(*module);
         std::copy(std::begin(memory_fill), std::end(memory_fill), std::begin(*instance->memory));
         EXPECT_THAT(execute(*instance, 0, {1}), Result(std::get<1>(test_case)));
@@ -418,7 +418,7 @@ TEST(execute, i64_load_all_variants)
         from_hex("0061736d0100000001060160017f017e030201000504010101010a0901070020002903000b");
     const auto module = parse(wasm);
 
-    auto& load_instr = const_cast<Instr&>(module->codesec[0].instructions[1]);
+    auto& load_instr = const_cast<uint8_t&>(module->codesec[0].instructions[1]);
     ASSERT_EQ(load_instr, Instr::i64_load);
     ASSERT_EQ(module->codesec[0].immediates.substr(4), "00000000"_bytes);  // load offset.
 
@@ -436,7 +436,7 @@ TEST(execute, i64_load_all_variants)
 
     for (const auto& test_case : test_cases)
     {
-        load_instr = std::get<0>(test_case);
+        load_instr = static_cast<uint8_t>(std::get<0>(test_case));
         auto instance = instantiate(*module);
         std::copy(std::begin(memory_fill), std::end(memory_fill), std::begin(*instance->memory));
         EXPECT_THAT(execute(*instance, 0, {1}), Result(std::get<1>(test_case)));
@@ -529,7 +529,7 @@ TEST(execute, i32_store_all_variants)
         from_hex("0061736d0100000001060160027f7f00030201000504010101010a0b010900200120003602000b");
     const auto module = parse(wasm);
 
-    auto& store_instr = const_cast<Instr&>(module->codesec[0].instructions[2]);
+    auto& store_instr = const_cast<uint8_t&>(module->codesec[0].instructions[2]);
     ASSERT_EQ(store_instr, Instr::i32_store);
     ASSERT_EQ(module->codesec[0].immediates.substr(8), "00000000"_bytes);  // store offset
 
@@ -541,7 +541,7 @@ TEST(execute, i32_store_all_variants)
 
     for (const auto& test_case : test_cases)
     {
-        store_instr = std::get<0>(test_case);
+        store_instr = static_cast<uint8_t>(std::get<0>(test_case));
         auto instance = instantiate(*module);
         std::fill_n(instance->memory->begin(), 6, uint8_t{0xcc});
         EXPECT_THAT(execute(*instance, 0, {0xb3b2b1b0, 1}), Result());
@@ -565,7 +565,7 @@ TEST(execute, i64_store_all_variants)
         from_hex("0061736d0100000001060160027e7f00030201000504010101010a0b010900200120003703000b");
     const auto module = parse(wasm);
 
-    auto& store_instr = const_cast<Instr&>(module->codesec[0].instructions[2]);
+    auto& store_instr = const_cast<uint8_t&>(module->codesec[0].instructions[2]);
     ASSERT_EQ(store_instr, Instr::i64_store);
     ASSERT_EQ(module->codesec[0].immediates.substr(8), "00000000"_bytes);  // store offset
 
@@ -578,7 +578,7 @@ TEST(execute, i64_store_all_variants)
 
     for (const auto& test_case : test_cases)
     {
-        store_instr = std::get<0>(test_case);
+        store_instr = static_cast<uint8_t>(std::get<0>(test_case));
         auto instance = instantiate(*module);
         std::fill_n(instance->memory->begin(), 10, uint8_t{0xcc});
         EXPECT_THAT(execute(*instance, 0, {0xb7b6b5b4b3b2b1b0, 1}), Result());
diff --git a/test/utils/asserts.hpp b/test/utils/asserts.hpp
index 058d986c31..3aa8e3f881 100644
--- a/test/utils/asserts.hpp
+++ b/test/utils/asserts.hpp
@@ -53,9 +53,15 @@ MATCHER_P(Result, value, "")  // NOLINT(readability-redundant-string-init)
 
 namespace fizzy
 {
-std::ostream& operator<<(std::ostream& os, ExecutionResult);
+/// Equal operator for Instr and bytes. Convenient for unit tests.
+inline constexpr bool operator==(uint8_t a, Instr b) noexcept
+{
+    return a == static_cast<uint8_t>(b);
 }
 
+std::ostream& operator<<(std::ostream& os, ExecutionResult);
+}  // namespace fizzy
+
 namespace fizzy::test
 {
 inline uint32_t as_uint32(fizzy::Value value)