diff --git a/src/c-kzg.h b/src/c-kzg.h
index bbc4978..4e16f61 100644
--- a/src/c-kzg.h
+++ b/src/c-kzg.h
@@ -14,6 +14,30 @@
  * limitations under the License.
  */
 
+#ifndef C_KZG_H
+#define C_KZG_H
+
+typedef enum {
+    C_KZG_SUCCESS = 0,
+    C_KZG_BADARGS,
+    c_KZG_ERROR,
+} C_KZG_RET;
+
 #include <stdbool.h>
-#include <assert.h>
 #include "../inc/blst.h"
+
+#define DEBUG
+
+#include <assert.h>
+#ifdef DEBUG
+#include <stdio.h>
+#define ASSERT(cond, ret) if (!(cond)) \
+        { \
+            printf("\n%s:%d: Failed ASSERT: %s\n", __FILE__, __LINE__, #cond); \
+            abort(); \
+        }
+#else
+#define ASSERT(cond, ret) if (!(cond)) return (ret)
+#endif
+
+#endif
diff --git a/src/fft_fr.c b/src/fft_fr.c
index f9391f6..64ad8ea 100644
--- a/src/fft_fr.c
+++ b/src/fft_fr.c
@@ -50,10 +50,10 @@ void fft_fr_fast(blst_fr *out, blst_fr *in, uint64_t stride, blst_fr *roots, uin
 }
 
 // The main entry point for forward and reverse FFTs
-void fft_fr (blst_fr *out, blst_fr *in, FFTSettings *fs, bool inv, uint64_t n) {
+C_KZG_RET fft_fr (blst_fr *out, blst_fr *in, FFTSettings *fs, bool inv, uint64_t n) {
     uint64_t stride = fs->max_width / n;
-    assert(n <= fs->max_width);
-    assert(is_power_of_two(n));
+    ASSERT(n <= fs->max_width, C_KZG_BADARGS);
+    ASSERT(is_power_of_two(n), C_KZG_BADARGS);
     if (inv) {
         blst_fr inv_len;
         fr_from_uint64(&inv_len, n);
@@ -65,4 +65,5 @@ void fft_fr (blst_fr *out, blst_fr *in, FFTSettings *fs, bool inv, uint64_t n) {
     } else {
         fft_fr_fast(out, in, 1, fs->expanded_roots_of_unity, stride, fs->max_width);
     }
+    return C_KZG_SUCCESS;
 }
diff --git a/src/fft_fr.h b/src/fft_fr.h
index 48f96d7..03cb147 100644
--- a/src/fft_fr.h
+++ b/src/fft_fr.h
@@ -19,4 +19,4 @@
 
 void fft_fr_slow(blst_fr *out, blst_fr *in, uint64_t stride, blst_fr *roots, uint64_t roots_stride, uint64_t l);
 void fft_fr_fast(blst_fr *out, blst_fr *in, uint64_t stride, blst_fr *roots, uint64_t roots_stride, uint64_t l);
-void fft_fr (blst_fr *out, blst_fr *in, FFTSettings *fs, bool inv, uint64_t n);
+C_KZG_RET fft_fr (blst_fr *out, blst_fr *in, FFTSettings *fs, bool inv, uint64_t n);
diff --git a/src/fft_fr_test.c b/src/fft_fr_test.c
index de45f7d..99ec96a 100644
--- a/src/fft_fr_test.c
+++ b/src/fft_fr_test.c
@@ -69,8 +69,8 @@ void roundtrip_fft(void) {
     }
 
     // Forward and reverse FFT
-    fft_fr(coeffs, data, &fs, false, fs.max_width);
-    fft_fr(data, coeffs, &fs, true, fs.max_width);
+    TEST_CHECK(fft_fr(coeffs, data, &fs, false, fs.max_width) == C_KZG_SUCCESS);
+    TEST_CHECK(fft_fr(data, coeffs, &fs, true, fs.max_width) == C_KZG_SUCCESS);
 
     // Verify that the result is still ascending values of i
     for (int i = 0; i < fs.max_width; i++) {
@@ -91,7 +91,7 @@ void inverse_fft(void) {
     }
 
     // Inverst FFT
-    fft_fr(out, data, &fs, true, fs.max_width);
+    TEST_CHECK(fft_fr(out, data, &fs, true, fs.max_width) == C_KZG_SUCCESS);
 
     // Verify against the known result, `inv_fft_expected`
     int n = sizeof(inv_fft_expected) / sizeof(inv_fft_expected[0]);
diff --git a/src/fft_g1.c b/src/fft_g1.c
index c1ed208..afed84f 100644
--- a/src/fft_g1.c
+++ b/src/fft_g1.c
@@ -63,10 +63,10 @@ void fft_g1_fast(blst_p1 *out, blst_p1 *in, uint64_t stride, blst_fr *roots, uin
 }
 
 // The main entry point for forward and reverse FFTs
-void fft_g1 (blst_p1 *out, blst_p1 *in, FFTSettings *fs, bool inv, uint64_t n) {
+C_KZG_RET fft_g1 (blst_p1 *out, blst_p1 *in, FFTSettings *fs, bool inv, uint64_t n) {
     uint64_t stride = fs->max_width / n;
-    assert(n <= fs->max_width);
-    assert(is_power_of_two(n));
+    ASSERT(n <= fs->max_width, C_KZG_BADARGS);
+    ASSERT(is_power_of_two(n), C_KZG_BADARGS);
     if (inv) {
         blst_fr inv_len;
         fr_from_uint64(&inv_len, n);
@@ -78,4 +78,5 @@ void fft_g1 (blst_p1 *out, blst_p1 *in, FFTSettings *fs, bool inv, uint64_t n) {
     } else {
         fft_g1_fast(out, in, 1, fs->expanded_roots_of_unity, stride, fs->max_width);
     }
+    return C_KZG_SUCCESS;
 }
diff --git a/src/fft_g1.h b/src/fft_g1.h
index b5ffbd6..dde2bda 100644
--- a/src/fft_g1.h
+++ b/src/fft_g1.h
@@ -21,4 +21,4 @@ void p1_mul(blst_p1 *out, const blst_p1 *a, const blst_fr *b);
 void p1_sub(blst_p1 *out, const blst_p1 *a, const blst_p1 *b);
 void fft_g1_slow(blst_p1 *out, blst_p1 *in, uint64_t stride, blst_fr *roots, uint64_t roots_stride, uint64_t l);
 void fft_g1_fast(blst_p1 *out, blst_p1 *in, uint64_t stride, blst_fr *roots, uint64_t roots_stride, uint64_t l);
-void fft_g1 (blst_p1 *out, blst_p1 *in, FFTSettings *fs, bool inv, uint64_t n);
+C_KZG_RET fft_g1 (blst_p1 *out, blst_p1 *in, FFTSettings *fs, bool inv, uint64_t n);
diff --git a/src/fft_g1_test.c b/src/fft_g1_test.c
index adc360e..35b74ad 100644
--- a/src/fft_g1_test.c
+++ b/src/fft_g1_test.c
@@ -84,8 +84,8 @@ void roundtrip_fft(void) {
     make_data(data, fs.max_width);
     
     // Forward and reverse FFT
-    fft_g1(coeffs, data, &fs, false, fs.max_width);
-    fft_g1(data, coeffs, &fs, true, fs.max_width);
+    TEST_CHECK(fft_g1(coeffs, data, &fs, false, fs.max_width) == C_KZG_SUCCESS);
+    TEST_CHECK(fft_g1(data, coeffs, &fs, true, fs.max_width) == C_KZG_SUCCESS);
 
     // Verify that the result is still ascending values of i
     for (int i = 0; i < fs.max_width; i++) {
diff --git a/src/fft_util.c b/src/fft_util.c
index b926f38..e273afb 100644
--- a/src/fft_util.c
+++ b/src/fft_util.c
@@ -39,6 +39,7 @@ blst_fr *expand_root_of_unity(blst_fr *root_of_unity, uint64_t width) {
     roots[1] = *root_of_unity;
 
     for (int i = 2; !is_one(&roots[i - 1]); i++) {
+        //ASSERT(i <= width, C_KZG_ERROR);
         assert(i <= width);
         blst_fr_mul(&roots[i], &roots[i - 1], root_of_unity);
     }