diff --git a/crypto_kem/frodokem640aes/opt/matrix_aes.c b/crypto_kem/frodokem640aes/opt/matrix_aes.c
index d7993e5c..24372876 100644
--- a/crypto_kem/frodokem640aes/opt/matrix_aes.c
+++ b/crypto_kem/frodokem640aes/opt/matrix_aes.c
@@ -17,7 +17,8 @@ int PQCLEAN_FRODOKEM640AES_OPT_mul_add_as_plus_e(uint16_t *out, const uint16_t *
     // Generate-and-multiply: generate matrix A (N x N) row-wise, multiply by s on the right.
     // Inputs: s, e (N x N_BAR)
     // Output: out = A*s + e (N x N_BAR)
-    int i, j, k;
+    int k;
+    uint16_t i, j;
     int16_t a_row[4 * PARAMS_N];
 
     for (i = 0; i < (PARAMS_N * PARAMS_NBAR); i += 2) {
@@ -72,8 +73,8 @@ int PQCLEAN_FRODOKEM640AES_OPT_mul_add_sa_plus_e(uint16_t *out, const uint16_t *
     // Generate-and-multiply: generate matrix A (N x N) column-wise, multiply by s' on the left.
     // Inputs: s', e' (N_BAR x N)
     // Output: out = s'*A + e' (N_BAR x N)
-    int i, j, kk;
-
+    int j;
+    uint16_t i, kk;
     for (i = 0; i < (PARAMS_N * PARAMS_NBAR); i += 2) {
         *((uint32_t *)&out[i]) = *((uint32_t *)&e[i]);
     }