diff --git a/ckiwi/ckiwi.cpp b/ckiwi/ckiwi.cpp
index b0247c9..476066c 100644
--- a/ckiwi/ckiwi.cpp
+++ b/ckiwi/ckiwi.cpp
@@ -55,44 +55,26 @@ const KiwiErr* wrap_err(F&& f) {
    try {
       f();
    } catch (const UnsatisfiableConstraint& ex) {
-      static const constexpr KiwiErr err {
-          KiwiErrUnsatisfiableConstraint,
-          "The constraint cannot be satisfied."
-      };
+      static const constexpr KiwiErr err {KiwiErrUnsatisfiableConstraint};
       return &err;
    } catch (const UnknownConstraint& ex) {
-      static const constexpr KiwiErr err {
-          KiwiErrUnknownConstraint,
-          "The constraint has not been added to the solver."
-      };
+      static const constexpr KiwiErr err {KiwiErrUnknownConstraint};
       return &err;
 
    } catch (const DuplicateConstraint& ex) {
-      static const constexpr KiwiErr err {
-          KiwiErrDuplicateConstraint,
-          "The constraint has already been added to the solver."
-      };
+      static const constexpr KiwiErr err {KiwiErrDuplicateConstraint};
       return &err;
 
    } catch (const UnknownEditVariable& ex) {
-      static const constexpr KiwiErr err {
-          KiwiErrUnknownEditVar,
-          "The edit variable has not been added to the solver."
-      };
+      static const constexpr KiwiErr err {KiwiErrUnknownEditVar};
       return &err;
 
    } catch (const DuplicateEditVariable& ex) {
-      static const constexpr KiwiErr err {
-          KiwiErrDuplicateEditVar,
-          "The edit variable has already been added to the solver."
-      };
+      static const constexpr KiwiErr err {KiwiErrDuplicateEditVar};
       return &err;
 
    } catch (const BadRequiredStrength& ex) {
-      static const constexpr KiwiErr err {
-          KiwiErrBadRequiredStrength,
-          "A required strength cannot be used in this context."
-      };
+      static const constexpr KiwiErr err {KiwiErrBadRequiredStrength};
       return &err;
 
    } catch (const InternalSolverError& ex) {
@@ -161,9 +143,9 @@ void kiwi_str_release(char* str) {
       std::free(str);
 }
 
-void kiwi_err_release(KiwiErr* err) {
+void kiwi_err_release(const KiwiErr* err) {
    if (lk_likely(err && err->must_release))
-      std::free(err);
+      std::free(const_cast<KiwiErr*>(err));
 }
 
 KiwiVar* kiwi_var_new(const char* name) {
diff --git a/ckiwi/ckiwi.h b/ckiwi/ckiwi.h
index 88f2d98..39477a8 100644
--- a/ckiwi/ckiwi.h
+++ b/ckiwi/ckiwi.h
@@ -78,7 +78,7 @@ struct KiwiSolver;
 LJKIWI_EXP void kiwi_solver_type_info(unsigned sz_align[2]);
 
 LJKIWI_EXP void kiwi_str_release(char* str);
-LJKIWI_EXP void kiwi_err_release(KiwiErr* err);
+LJKIWI_EXP void kiwi_err_release(const KiwiErr* err);
 
 LJKIWI_EXP KiwiVar* kiwi_var_new(const char* name);
 LJKIWI_EXP void kiwi_var_release(KiwiVar* var);
diff --git a/kiwi.lua b/kiwi.lua
index 4673de9..e7dc66f 100644
--- a/kiwi.lua
+++ b/kiwi.lua
@@ -110,7 +110,7 @@ typedef struct KiwiErr {
 struct KiwiSolver;
 
 void kiwi_str_release(char *);
-void kiwi_err_release(KiwiErr *);
+void kiwi_err_release(const KiwiErr *);
 
 KiwiVar* kiwi_var_new(const char* name);
 void kiwi_var_release(KiwiVar* var);
@@ -438,8 +438,7 @@ do
 
    if RUST then
       function Var_mt:__new(name)
-         --return ffi_gc(ljkiwi.kiwi_var_new(name)[0], ljkiwi.kiwi_var_release)
-         return ljkiwi.kiwi_var_new(name)[0]
+         return ffi_gc(ljkiwi.kiwi_var_new(name)[0], ljkiwi.kiwi_var_release)
       end
 
       --- Get the name of the variable.
@@ -1020,6 +1019,18 @@ do
       }, Error_mt)
    end
 
+   local ERR_MESSAGES = {
+      "The constraint cannot be satisfied.",
+      "The constraint has not been added to the solver.",
+      "The constraint has already been added to the solver.",
+      "The edit variable has not been added to the solver.",
+      "The edit variable has already been added to the solver.",
+      "A required strength cannot be used in this context.",
+      "An internal solver error occurred.",
+      "A memory allocation failed.",
+      "null object passed as argument.",
+      "An unknown error occurred.",
+   }
    ---@generic T
    ---@param f fun(solver: kiwi.Solver, item: T, ...): kiwi.KiwiErr?
    ---@param solver kiwi.Solver
@@ -1029,10 +1040,12 @@ do
       local err = f(solver, item, ...)
       if err ~= nil then
          if err.must_release then
-            ffi_gc(err, ljkiwi.kiwi_error_release)
+            ffi_gc(err, ljkiwi.kiwi_err_release)
          end
          local kind = err.kind
-         local message = err.message ~= nil and ffi_string(err.message) or ""
+         local message = err.message ~= nil and ffi_string(err.message)
+            or ERR_MESSAGES[tonumber(err.kind)]
+            or ""
          local errdata = new_error(kind, message, solver, item)
          local error_mask = ljkiwi.kiwi_solver_get_error_mask(solver)
          return item,