diff --git a/lib/fizzy/execute.cpp b/lib/fizzy/execute.cpp
index 10ffe76733..5eb860c910 100644
--- a/lib/fizzy/execute.cpp
+++ b/lib/fizzy/execute.cpp
@@ -510,8 +510,8 @@ void branch(const Code& code, OperandStack& stack, const uint8_t*& pc, uint32_t
         stack.drop(stack_drop);
 }
 
-inline bool invoke_function(size_t num_args, uint32_t func_idx, Instance& instance,
-    OperandStack& stack, ExecutionContext& ctx) noexcept
+inline bool invoke_function(size_t num_args, [[maybe_unused]] size_t num_ret, uint32_t func_idx,
+    Instance& instance, OperandStack& stack, ExecutionContext& ctx) noexcept
 {
     assert(stack.size() >= num_args);
     const auto call_args = stack.rend() - num_args;
@@ -524,6 +524,7 @@ inline bool invoke_function(size_t num_args, uint32_t func_idx, Instance& instan
     stack.drop(num_args);
 
     // NOTE: validation ensures there is at most 1 output value
+    assert(num_ret == (ret.has_value ? 1 : 0));
     // Push back the result
     if (ret.has_value)
         stack.push(ret.value);
@@ -627,10 +628,10 @@ ExecutionResult execute(
         case Instr::call:
         {
             const auto called_func_idx = read<uint32_t>(pc);
-            const auto called_func_num_args =
-                instance.module->get_function_type(called_func_idx).inputs.size();
+            const auto called_func = instance.module->get_function_type(called_func_idx);
 
-            if (!invoke_function(called_func_num_args, called_func_idx, instance, stack, ctx))
+            if (!invoke_function(called_func.inputs.size(), called_func.outputs.size(),
+                    called_func_idx, instance, stack, ctx))
                 goto trap;
             break;
         }
@@ -656,8 +657,8 @@ ExecutionResult execute(
             if (expected_type != actual_type)
                 goto trap;
 
-            if (!invoke_function(actual_type.inputs.size(), called_func.func_idx,
-                    *called_func.instance, stack, ctx))
+            if (!invoke_function(actual_type.inputs.size(), actual_type.outputs.size(),
+                    called_func.func_idx, *called_func.instance, stack, ctx))
                 goto trap;
             break;
         }