diff --git a/furvm/include/furvm/type.hpp b/furvm/include/furvm/type.hpp
index 6e18d46..d4d52d3 100644
--- a/furvm/include/furvm/type.hpp
+++ b/furvm/include/furvm/type.hpp
@@ -5,6 +5,7 @@
 #include "furvm/handle.hpp" // IWYU pragma: keep
 
 #include <cstdint>
+#include <stdexcept>
 
 namespace furvm {
 
@@ -29,11 +30,17 @@ struct array_type {
      * Size of the array. If size is equal to zero, then the array becomes dynamic.
      */
     std::size_t size;
+
+    bool operator==(const array_type& rhs) const { return type == rhs.type && size == rhs.size; }
+    bool operator!=(const array_type& rhs) const { return !this->operator==(rhs); }
 };
 
 struct import_type {
     mod_id  mod;
     type_id type;
+
+    bool operator==(const import_type& rhs) const { return mod == rhs.mod && type == rhs.type; }
+    bool operator!=(const import_type& rhs) const { return !this->operator==(rhs); }
 };
 
 struct type {
@@ -108,6 +115,18 @@ struct type {
 
         return *this;
     }
+
+    bool operator==(const type& rhs) const {
+        if (t != rhs.t) return false;
+        switch (t) {
+        case type_t::Primitive: return primitive == rhs.primitive;
+        case type_t::Array: return array == rhs.array;
+        case type_t::Import: return imp == rhs.imp;
+        }
+        throw std::runtime_error("unreachable");
+    }
+
+    bool operator!=(const type& rhs) const { return !this->operator==(rhs); }
 };
 
 using byte_t  = std::int8_t;  /**< A 1-byte integer. */
diff --git a/furvm/src/executor.cpp b/furvm/src/executor.cpp
index c74cb57..1cdec80 100644
--- a/furvm/src/executor.cpp
+++ b/furvm/src/executor.cpp
@@ -21,10 +21,14 @@ void executor::push_frame(const mod_h& mod, function function) {
         function = *modInst->function_at(function.imp().function);
     }
 
+    auto                 signature = function.signature();
     std::vector<thing_h> args;
-    args.reserve(function.signature().params.size());
-    while (args.size() < args.capacity())
-        args.push_back(pop_thing());
+    args.reserve(signature.params.size());
+    for (const auto& param : signature.params) {
+        auto arg = pop_thing();
+        if (arg->type().type != param) throw std::runtime_error("function argument type mismatch");
+        args.push_back(std::move(arg));
+    }
 
     switch (function.type()) {
     case function_t::Normal: {