diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 7d029de1fc..186dabef20 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -5211,36 +5211,39 @@ cleanup:
 }
 
 MBEDTLS_STATIC_TESTABLE
-int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *N_p, size_t N_n)
+int mbedtls_ecp_mod_p521_raw(mbedtls_mpi_uint *X, size_t X_limbs)
 {
     mbedtls_mpi_uint carry = 0;
 
-    if (N_n > 2 * P521_WIDTH - 1) {
-        N_n = 2 * P521_WIDTH - 1;
+    if (X_limbs > 2 * P521_WIDTH - 1) {
+        X_limbs = 2 * P521_WIDTH - 1;
     }
-    if (N_n < P521_WIDTH) {
+    if (X_limbs < P521_WIDTH) {
         return 0;
     }
 
-    if (N_n > P521_WIDTH) {
-        /* Helper references for top part of N */
-        mbedtls_mpi_uint *NT_p = N_p + P521_WIDTH;
-        size_t NT_n = N_n - P521_WIDTH;
+    if (X_limbs > P521_WIDTH) {
+        /* Helper references for bottom part of X */
+        mbedtls_mpi_uint *X0 = X;
+        size_t X0_limbs = P521_WIDTH;
+        /* Helper references for top part of X */
+        mbedtls_mpi_uint *X1 = X + X0_limbs;
+        size_t X1_limbs = X_limbs - X0_limbs;
 
-        /* Split N as A0 + 2^(512 + biL) A1 and compute A0 + 2^(biL - 9) * A1.
+        /* Split X as X0 + 2^(512 + biL) X1 and compute X0 + 2^(biL - 9) * X1.
          * This can be done in place. */
         mbedtls_mpi_uint shift = ((mbedtls_mpi_uint) 1u) << (biL - 9);
-        carry = mbedtls_mpi_core_mla(N_p, P521_WIDTH, NT_p, NT_n, shift);
+        carry = mbedtls_mpi_core_mla(X0, X0_limbs, X1, X1_limbs, shift);
 
         /* Clear top part */
-        memset(NT_p, 0, sizeof(mbedtls_mpi_uint) * NT_n);
+        memset(X1, 0, X1_limbs * sizeof(mbedtls_mpi_uint));
     }
 
-    mbedtls_mpi_uint remainder[P521_WIDTH] = { 0 };
-    remainder[0] = carry << (biL - 9);
-    remainder[0] += (N_p[P521_WIDTH - 1] >> 9);
-    N_p[P521_WIDTH - 1] &= P521_MASK;
-    (void) mbedtls_mpi_core_add(N_p, N_p, remainder, P521_WIDTH);
+    mbedtls_mpi_uint addend[P521_WIDTH] = { 0 };
+    addend[0] = carry << (biL - 9);
+    addend[0] += (X[P521_WIDTH - 1] >> 9);
+    X[P521_WIDTH - 1] &= P521_MASK;
+    (void) mbedtls_mpi_core_add(X, X, addend, P521_WIDTH);
 
     return 0;
 }