diff --git a/ChangeLog.d/mpi-add-0-ub.txt b/ChangeLog.d/mpi-add-0-ub.txt
new file mode 100644
index 000000000..9f131a430
--- /dev/null
+++ b/ChangeLog.d/mpi-add-0-ub.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix undefined behavior (typically harmless in practice) of
+     mbedtls_mpi_add_mpi(), mbedtls_mpi_add_abs() and mbedtls_mpi_add_int()
+     when both operands are 0 and the left operand is represented with 0 limbs.
diff --git a/library/bignum.c b/library/bignum.c
index 521787d74..497ccbc81 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -889,6 +889,11 @@ int mbedtls_mpi_add_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
         if( B->p[j - 1] != 0 )
             break;
 
+    /* Exit early to avoid undefined behavior on NULL+0 when X->n == 0
+     * and B is 0 (of any size). */
+    if( j == 0 )
+        return( 0 );
+
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, j ) );
 
     /* j is the number of non-zero limbs of B. Add those to X. */