diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index aa2bd46..b232182 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -128,6 +128,21 @@
                              N->rep.mont.mm, T);
 }
 
+MBEDTLS_STATIC_TESTABLE
+int mbedtls_mpi_mod_raw_fix_quasi_reduction(mbedtls_mpi_uint *X,
+                                            const mbedtls_mpi_mod_modulus *N)
+{
+    if (N->limbs == 0) {
+        return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+    }
+
+    mbedtls_mpi_uint c = mbedtls_mpi_core_sub(X, X, N->p, N->limbs);
+
+    (void) mbedtls_mpi_core_add_if(X, N->p, N->limbs, (unsigned) c);
+
+    return 0;
+}
+
 /* END MERGE SLOT 2 */
 
 /* BEGIN MERGE SLOT 3 */