diff --git a/library/bignum_core.c b/library/bignum_core.c
index 3e19ff4a5..a05df9946 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -185,28 +185,27 @@ int mbedtls_mpi_core_read_be( mbedtls_mpi_uint *X,
                               const unsigned char *buf,
                               size_t buflen )
 {
-    const size_t limbs = CHARS_TO_LIMBS( buflen );
+    size_t const limbs = CHARS_TO_LIMBS( buflen );
 
     if( nx < limbs )
         return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
 
-    if( X != NULL )
+    /* If nx is 0, buflen must also be 0 (from previous test). Nothing to do. */
+    if( nx == 0 )
+        return( 0 );
+
+    memset( X, 0, nx * ciL );
+
+    /* memcpy() with (NULL, 0) is undefined behaviour */
+    if( buflen != 0 )
     {
-        memset( X, 0, nx * ciL );
-
-        const size_t overhead = ( nx * ciL ) - buflen;
-
-        /* Avoid calling `memcpy` with NULL source or destination argument,
-         * even if buflen is 0. */
-        if( buf != NULL )
-        {
-            unsigned char *Xp = (unsigned char *) X;
-            memcpy( Xp + overhead, buf, buflen );
-
-            mbedtls_mpi_core_bigendian_to_host( X, nx );
-        }
+        size_t overhead = ( nx * ciL ) - buflen;
+        unsigned char *Xp = (unsigned char *) X;
+        memcpy( Xp + overhead, buf, buflen );
     }
 
+    mbedtls_mpi_core_bigendian_to_host( X, nx );
+
     return( 0 );
 }
 
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 117c3c5f3..397f79a47 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -85,8 +85,10 @@ int mbedtls_mpi_core_read_le( mbedtls_mpi_uint *X,
  * most significant zero bytes in the input).
  *
  * \param X      The address of the MPI.
+ *               May only be #NULL if \nx is 0 and \p buflen is 0.
  * \param nx     The number of limbs of \p X.
  * \param buf    The input buffer to import from.
+ *               May only be #NULL if \p buflen is 0.
  * \param buflen The length in bytes of \p buf.
  *
  * \return       \c 0 if successful.