diff --git a/ckiwi/ckiwi.cpp b/ckiwi/ckiwi.cpp
index 07bbe3f..56c232c 100644
--- a/ckiwi/ckiwi.cpp
+++ b/ckiwi/ckiwi.cpp
@@ -2,6 +2,7 @@
 
 #include <kiwi/kiwi.h>
 
+#include <algorithm>
 #include <climits>
 #include <cstdlib>
 #include <cstring>
@@ -181,7 +182,7 @@ void kiwi_var_set_value(KiwiVar* var, double value) {
 void kiwi_expression_retain(KiwiExpression* expr) {
    if (lk_unlikely(!expr))
       return;
-   for (auto* t = expr->terms_; t != expr->terms_ + expr->term_count; ++t) {
+   for (auto* t = expr->terms_; t != expr->terms_ + std::max(expr->term_count, 0); ++t) {
       retain_unmanaged(t->var);
    }
    expr->owner = expr;
@@ -192,7 +193,7 @@ void kiwi_expression_destroy(KiwiExpression* expr) {
       return;
 
    if (expr->owner == expr) {
-      for (auto* t = expr->terms_; t != expr->terms_ + expr->term_count; ++t) {
+      for (auto* t = expr->terms_; t != expr->terms_ + std::max(expr->term_count, 0); ++t) {
          release_unmanaged(t->var);
       }
    } else {
@@ -200,6 +201,51 @@ void kiwi_expression_destroy(KiwiExpression* expr) {
    }
 }
 
+void kiwi_expression_add_term(
+    const KiwiExpression* expr,
+    KiwiVar* var,
+    double coefficient,
+    KiwiExpression* out
+) {
+   if (lk_unlikely(!expr || expr->term_count == INT_MAX || expr->term_count < 0)) {
+      out->term_count = 0;
+      return;
+   }
+
+   out->owner = out;
+   out->term_count = expr->term_count + 1;
+   out->constant = expr->constant;
+
+   auto* d = out->terms_;
+   for (auto* s = expr->terms_; s != expr->terms_ + expr->term_count; ++s, ++d) {
+      d->var = retain_unmanaged(s->var);
+      d->coefficient = s->coefficient;
+   }
+   d->var = retain_unmanaged(var);
+   d->coefficient = coefficient;
+}
+
+void kiwi_expression_set_constant(
+    const KiwiExpression* expr,
+    double constant,
+    KiwiExpression* out
+) {
+   if (lk_unlikely(!expr || expr->term_count < 0)) {
+      out->term_count = 0;
+      return;
+   }
+
+   out->owner = out;
+   out->term_count = expr->term_count;
+   out->constant = constant;
+
+   auto* d = out->terms_;
+   for (auto* s = expr->terms_; s != expr->terms_ + expr->term_count; ++s, ++d) {
+      d->var = retain_unmanaged(s->var);
+      d->coefficient = s->coefficient;
+   }
+}
+
 KiwiConstraint* kiwi_constraint_new(
     const KiwiExpression* lhs,
     const KiwiExpression* rhs,
diff --git a/ckiwi/ckiwi.h b/ckiwi/ckiwi.h
index 15d2213..e3e914e 100644
--- a/ckiwi/ckiwi.h
+++ b/ckiwi/ckiwi.h
@@ -91,6 +91,14 @@ LJKIWI_EXP void kiwi_var_set_value(KiwiVar* var, double value);
 
 LJKIWI_EXP void kiwi_expression_retain(KiwiExpression* expr);
 LJKIWI_EXP void kiwi_expression_destroy(KiwiExpression* expr);
+LJKIWI_EXP void kiwi_expression_add_term(
+    const KiwiExpression* expr,
+    KiwiVar* var,
+    double coefficient,
+    KiwiExpression* out
+);
+LJKIWI_EXP void
+kiwi_expression_set_constant(const KiwiExpression* expr, double constant, KiwiExpression* out);
 
 LJKIWI_EXP KiwiConstraint* kiwi_constraint_new(
     const KiwiExpression* lhs,
diff --git a/kiwi.lua b/kiwi.lua
index 7c03db2..97fb437 100644
--- a/kiwi.lua
+++ b/kiwi.lua
@@ -63,7 +63,8 @@ typedef struct KiwiExpression {
 
 void kiwi_expression_retain(KiwiExpression* expr);
 void kiwi_expression_destroy(KiwiExpression* expr);
-
+void kiwi_expression_add_term(const KiwiExpression* expr, KiwiVar* var, double coeff, KiwiExpression* out);
+void kiwi_expression_set_constant(const KiwiExpression* expr, double constant, KiwiExpression* out);
 ]])
 
 if RUST then
@@ -95,9 +96,6 @@ struct KiwiVar {
 };
 
 void kiwi_var_free(KiwiVar* var);
-
-void kiwi_expression_add_term(const KiwiExpression* expr, KiwiVar* var, double coeff, KiwiExpression* out);
-void kiwi_expression_set_constant(const KiwiExpression* expr, double constant, KiwiExpression* out);
 ]])
 else
    ffi.cdef([[
@@ -298,29 +296,14 @@ else
    end
 end
 
-local add_expr_term
-if RUST then
-   ---@param expr kiwi.Expression
-   ---@param var kiwi.Var
-   ---@param coeff number?
-   ---@nodiscard
-   function add_expr_term(expr, var, coeff)
-      local ret = ffi_new(Expression, expr.term_count + 1)
-      ljkiwi.kiwi_expression_add_term(expr, var, coeff or 1.0, ret)
-      return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
-   end
-else
-   function add_expr_term(expr, var, coeff)
-      local ret = ffi_new(Expression, expr.term_count + 1) --[[@as kiwi.Expression]]
-      ffi_copy(ret.terms_, expr.terms_, SIZEOF_TERM * expr.term_count)
-      local dt = ret.terms_[expr.term_count]
-      dt.var = var
-      dt.coefficient = coeff or 1.0
-      ret.constant = expr.constant
-      ret.term_count = expr.term_count + 1
-      ljkiwi.kiwi_expression_retain(ret)
-      return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
-   end
+---@param expr kiwi.Expression
+---@param var kiwi.Var
+---@param coeff number?
+---@nodiscard
+local function add_expr_term(expr, var, coeff)
+   local ret = ffi_new(Expression, expr.term_count + 1)
+   ljkiwi.kiwi_expression_add_term(expr, var, coeff or 1.0, ret)
+   return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
 end
 
 ---@param constant number
@@ -738,25 +721,13 @@ do
       return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
    end
 
-   local new_expr_constant
-   if RUST then
-      ---@param expr kiwi.Expression
-      ---@param constant number
-      ---@nodiscard
-      function new_expr_constant(expr, constant)
-         local ret = ffi_new(Expression, expr.term_count)
-         ljkiwi.kiwi_expression_set_constant(expr, constant, ret)
-         return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
-      end
-   else
-      function new_expr_constant(expr, constant)
-         local ret = ffi_new(Expression, expr.term_count) --[[@as kiwi.Expression]]
-         ffi_copy(ret.terms_, expr.terms_, SIZEOF_TERM * expr.term_count)
-         ret.constant = constant
-         ret.term_count = expr.term_count
-         ljkiwi.kiwi_expression_retain(ret)
-         return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
-      end
+   ---@param expr kiwi.Expression
+   ---@param constant number
+   ---@nodiscard
+   local function new_expr_constant(expr, constant)
+      local ret = ffi_new(Expression, expr.term_count)
+      ljkiwi.kiwi_expression_set_constant(expr, constant, ret)
+      return ffi_gc(ret, ljkiwi.kiwi_expression_destroy) --[[@as kiwi.Expression]]
    end
 
    ---@return number
diff --git a/rjkiwi/src/expr.rs b/rjkiwi/src/expr.rs
index 839b524..3eb90cf 100644
--- a/rjkiwi/src/expr.rs
+++ b/rjkiwi/src/expr.rs
@@ -40,7 +40,7 @@ impl KiwiExpression {
     }
 
     unsafe fn try_from_raw<'a>(e: *const KiwiExpressionPtr) -> Option<&'a Self> {
-        if e.is_null() {
+        if e.is_null() || (*e).term_count < 0 {
             None
         } else {
             Some(Self::from_raw(e))
@@ -56,10 +56,15 @@ pub unsafe extern "C" fn kiwi_expression_add_term(
     out: *mut KiwiExpressionPtr,
 ) {
     let Some(e) = KiwiExpression::try_from_raw(e) else {
+        (*out).term_count = 0;
         return;
     };
+    if e.terms_.len() >= c_int::MAX as usize {
+        (*out).term_count = 0;
+        return;
+    }
 
-    let n_terms = (e.terms_.len() + 1).min(c_int::MAX as usize);
+    let n_terms = e.terms_.len() + 1;
 
     let out = core::slice::from_raw_parts_mut(out as *mut (), n_terms) as *mut [()]
         as *mut KiwiExpression;
@@ -85,6 +90,7 @@ pub unsafe extern "C" fn kiwi_expression_set_constant(
     out: *mut KiwiExpressionPtr,
 ) {
     let Some(e) = KiwiExpression::try_from_raw(e) else {
+        (*out).term_count = 0;
         return;
     };
 
diff --git a/rjkiwi/src/lib.rs b/rjkiwi/src/lib.rs
index 71a8e3c..88ee656 100644
--- a/rjkiwi/src/lib.rs
+++ b/rjkiwi/src/lib.rs
@@ -7,7 +7,7 @@ pub mod solver;
 mod util;
 pub mod var;
 
-mod mem {
+pub mod mem {
     use std::{
         ffi::{c_char, CString},
         ptr::NonNull,
@@ -20,3 +20,10 @@ mod mem {
         }
     }
 }
+
+pub mod ffi {
+    pub use crate::expr::*;
+    pub use crate::mem::*;
+    pub use crate::solver::*;
+    pub use crate::var::*;
+}