diff --git a/library/bignum.c b/library/bignum.c
index 0d0d922c1..f968a0ad7 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -380,7 +380,7 @@ cleanup:
 size_t mbedtls_mpi_lsb( const mbedtls_mpi *X )
 {
     size_t i, j, count = 0;
-    MPI_VALIDATE_RET( X != NULL );
+    MBEDTLS_INTERNAL_VALIDATE_RET( X != NULL, 0 );
 
     for( i = 0; i < X->n; i++ )
         for( j = 0; j < biL; j++, count++ )
diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index 3379a999d..d1fa5a46c 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -216,6 +216,8 @@ void mpi_invalid_param( )
     TEST_INVALID_PARAM_RET( MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                             mbedtls_mpi_div_int( &X, &X, NULL, 42 ) );
 
+    TEST_INVALID_PARAM_RET( 0, mbedtls_mpi_lsb( NULL ) );
+
     TEST_INVALID_PARAM_RET( MBEDTLS_ERR_MPI_BAD_INPUT_DATA,
                             mbedtls_mpi_mod_mpi( NULL, &X, &X ) );
     TEST_INVALID_PARAM_RET( MBEDTLS_ERR_MPI_BAD_INPUT_DATA,