diff --git a/lib/fizzy/execute.cpp b/lib/fizzy/execute.cpp
index 1dc71f83ae..8313a2538a 100644
--- a/lib/fizzy/execute.cpp
+++ b/lib/fizzy/execute.cpp
@@ -302,11 +302,11 @@ bool invoke_function(
 
     const auto num_outputs = func_type.outputs.size();
     // NOTE: we can assume these two from validation
-    assert(ret.stack.size() == num_outputs);
     assert(num_outputs <= 1);
+    assert(ret.result.has_value() == num_outputs);
     // Push back the result
     if (num_outputs != 0)
-        stack.push(ret.stack[0]);
+        stack.push(*ret.result);
 
     return true;
 }
@@ -1567,8 +1567,10 @@ execution_result execute(
 
 end:
     assert(labels.empty() || trap);
-    // move allows to return derived Stack<uint64_t> instance into base vector<uint64_t> value
-    return {trap, std::move(stack)};
+    if (stack.empty())
+        return {trap, std::nullopt};
+    else
+        return {trap, stack.pop()};
 }
 
 execution_result execute(const Module& module, FuncIdx func_idx, std::vector<uint64_t> args)
diff --git a/lib/fizzy/execute.hpp b/lib/fizzy/execute.hpp
index a74df1ca17..884da231f3 100644
--- a/lib/fizzy/execute.hpp
+++ b/lib/fizzy/execute.hpp
@@ -19,7 +19,7 @@ struct execution_result
     bool trapped;
     // the resulting stack (e.g. return values)
     // NOTE: this can be either 0 or 1 items
-    std::vector<uint64_t> stack;
+    std::optional<uint64_t> result;
 };
 
 struct Instance;
diff --git a/test/spectests/spectests.cpp b/test/spectests/spectests.cpp
index 8eefa155fc..6042035c8b 100644
--- a/test/spectests/spectests.cpp
+++ b/test/spectests/spectests.cpp
@@ -210,20 +210,20 @@ class test_runner
                     const auto& expected = cmd.at("expected");
                     if (expected.empty())
                     {
-                        if (result->stack.empty())
+                        if (!result->result.has_value())
                             pass();
                         else
                             fail("Unexpected returned value.");
                         continue;
                     }
 
-                    if (result->stack.size() != 1)
+                    if (!result->result.has_value())
                     {
                         fail("More than 1 value returned.");
                         continue;
                     }
 
-                    if (!check_result(result->stack[0], expected.at(0)))
+                    if (!check_result(*result->result, expected.at(0)))
                         continue;
 
                     pass();
diff --git a/test/unittests/instantiate_test.cpp b/test/unittests/instantiate_test.cpp
index fc237d870d..926abde43b 100644
--- a/test/unittests/instantiate_test.cpp
+++ b/test/unittests/instantiate_test.cpp
@@ -17,7 +17,8 @@ uint64_t call_table_func(Instance& instance, size_t idx)
 {
     const auto& elem = (*instance.table)[idx];
     const auto res = elem->function(instance, {}, 0);
-    return res.stack.front();
+    // ASSERT_TRUE(res.result.has_value());
+    return *res.result;
 }
 }  // namespace
 
diff --git a/test/utils/asserts.cpp b/test/utils/asserts.cpp
index c5ffae3b9b..c2658f6433 100644
--- a/test/utils/asserts.cpp
+++ b/test/utils/asserts.cpp
@@ -13,12 +13,8 @@ std::ostream& operator<<(std::ostream& os, execution_result result)
         return os << "trapped";
 
     os << "result(";
-    std::string_view separator;
-    for (const auto& x : result.stack)
-    {
-        os << x << separator;
-        separator = ", ";
-    }
+    if (result.result.has_value())
+       os << *result.result;
     os << ")";
     return os;
 }
diff --git a/test/utils/asserts.hpp b/test/utils/asserts.hpp
index 79ff674697..479e90329f 100644
--- a/test/utils/asserts.hpp
+++ b/test/utils/asserts.hpp
@@ -15,12 +15,12 @@ MATCHER(Traps, "")
 
 MATCHER(Result, "empty result")
 {
-    return !arg.trapped && arg.stack.size() == 0;
+    return !arg.trapped && !arg.result.has_value();
 }
 
 MATCHER_P(Result, value, "")
 {
-    return !arg.trapped && arg.stack.size() == 1 && arg.stack[0] == uint64_t(value);
+    return !arg.trapped && arg.result.has_value() && *arg.result == uint64_t(value);
 }
 
 #define EXPECT_THROW_MESSAGE(stmt, ex_type, expected)                                        \
diff --git a/test/utils/fizzy_engine.cpp b/test/utils/fizzy_engine.cpp
index 18d0a3435e..61fa532e1f 100644
--- a/test/utils/fizzy_engine.cpp
+++ b/test/utils/fizzy_engine.cpp
@@ -84,9 +84,8 @@ std::optional<WasmEngine::FuncRef> FizzyEngine::find_function(std::string_view n
 WasmEngine::Result FizzyEngine::execute(
     WasmEngine::FuncRef func_ref, const std::vector<uint64_t>& args)
 {
-    const auto [trapped, result_stack] =
+    const auto [trapped, result] =
         fizzy::execute(*m_instance, static_cast<uint32_t>(func_ref), args);
-    assert(result_stack.size() <= 1);
-    return {trapped, !result_stack.empty() ? result_stack.back() : std::optional<uint64_t>{}};
+    return {trapped, result};
 }
 }  // namespace fizzy::test