diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index 1e4ebae..9de9827 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -6,6 +6,18 @@
 #define MPI_MAX_BITS_LARGER_THAN_792
 #endif
 
+/* Check the validity of the sign bit in an MPI object. Reject representations
+ * that are not supported by the rest of the library and indicate a bug when
+ * constructing the value. */
+static int sign_is_valid( const mbedtls_mpi *X )
+{
+    if( X->s != 1 && X->s != -1 )
+        return( 0 ); // invalid sign bit, e.g. 0
+    if( mbedtls_mpi_bitlen( X ) == 0 && X->s != 1 )
+        return( 0 ); // negative zero
+    return( 1 );
+}
+
 typedef struct mbedtls_test_mpi_random
 {
     data_t *data;
@@ -320,6 +332,7 @@
     TEST_ASSERT( mbedtls_mpi_read_string( &X, radix_X, input_X ) == result_read );
     if( result_read == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &X ) );
         TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, str, output_size, &len ) == result_write );
         if( result_write == 0 )
         {
@@ -344,6 +357,7 @@
 
 
     TEST_ASSERT( mbedtls_mpi_read_binary( &X, buf->x, buf->len ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, (char *) str, sizeof( str ), &len ) == 0 );
     TEST_ASSERT( strcmp( (char *) str, input_A ) == 0 );
 
@@ -406,6 +420,7 @@
 
     if( result == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &X ) );
         buflen = mbedtls_mpi_size( &X );
         TEST_ASSERT( mbedtls_mpi_write_binary( &X, buf, buflen ) == 0 );
 
@@ -476,6 +491,7 @@
 
     if( result == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &X ) );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &Y ) == 0 );
     }
 
@@ -523,6 +539,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &Y, radix_Y, input_Y ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_gcd( &Z, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
 exit:
@@ -611,6 +628,7 @@
     TEST_ASSERT( mbedtls_mpi_lset( &Y, input_Y ) == 0 );
 
     TEST_ASSERT( mbedtls_mpi_copy( &Y, &X ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_int( &X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_int( &Y, input_X ) == 0 );
 
@@ -631,6 +649,7 @@
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &X0 ) == 0 );
 
     TEST_ASSERT( mbedtls_mpi_copy( &Y, &X ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &X0 ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &X0 ) == 0 );
 
@@ -685,9 +704,11 @@
     TEST_ASSERT( mbedtls_mpi_copy( &XX, &X ) == 0 );
 
     TEST_ASSERT( mbedtls_mpi_safe_cond_assign( &X, &Y, 0 ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &XX ) == 0 );
 
     TEST_ASSERT( mbedtls_mpi_safe_cond_assign( &X, &Y, 1 ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &Y ) == 0 );
 
 exit:
@@ -713,10 +734,14 @@
     TEST_ASSERT( mbedtls_mpi_copy( &YY, &Y ) == 0 );
 
     TEST_ASSERT( mbedtls_mpi_safe_cond_swap( &X, &Y, 0 ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &XX ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &YY ) == 0 );
 
     TEST_ASSERT( mbedtls_mpi_safe_cond_swap( &X, &Y, 1 ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &XX ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &YY ) == 0 );
 
@@ -738,6 +763,8 @@
     TEST_ASSERT( mbedtls_mpi_cmp_int( &Y, input_Y ) == 0 );
 
     mbedtls_mpi_swap( &X, &Y );
+    TEST_ASSERT( sign_is_valid( &X ) );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_int( &X, input_Y ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_int( &Y, input_X ) == 0 );
 
@@ -759,6 +786,8 @@
     TEST_ASSERT( mbedtls_mpi_read_binary( &Y0, input_Y->x, input_Y->len ) == 0 );
 
     mbedtls_mpi_swap( &X, &Y );
+    TEST_ASSERT( sign_is_valid( &X ) );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &Y0 ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &X0 ) == 0 );
 
@@ -778,6 +807,7 @@
     TEST_ASSERT( mbedtls_mpi_read_binary( &X0, input_X->x, input_X->len ) == 0 );
 
     mbedtls_mpi_swap( &X, &X );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &X0 ) == 0 );
 
 exit:
@@ -796,15 +826,18 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &Y, radix_Y, input_Y ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_add_mpi( &Z, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
     /* result == first operand */
     TEST_ASSERT( mbedtls_mpi_add_mpi( &X, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
 
     /* result == second operand */
     TEST_ASSERT( mbedtls_mpi_add_mpi( &Y, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &A ) == 0 );
 
 exit:
@@ -824,13 +857,16 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_mpi_sub_abs( &X, &X, &X ) == 0 );
     TEST_ASSERT( mbedtls_mpi_cmp_int( &X, 0 ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
 
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_mpi_add_abs( &X, &X, &X ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
 
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_mpi_add_mpi( &X, &X, &X ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
 
 exit:
@@ -850,15 +886,18 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &Y, radix_Y, input_Y ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_add_abs( &Z, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
     /* result == first operand */
     TEST_ASSERT( mbedtls_mpi_add_abs( &X, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
 
     /* result == second operand */
     TEST_ASSERT( mbedtls_mpi_add_abs( &Y, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &A ) == 0 );
 
 exit:
@@ -876,6 +915,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_add_int( &Z, &X, input_Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
 exit:
@@ -894,15 +934,18 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &Y, radix_Y, input_Y ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_sub_mpi( &Z, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
     /* result == first operand */
     TEST_ASSERT( mbedtls_mpi_sub_mpi( &X, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
 
     /* result == second operand */
     TEST_ASSERT( mbedtls_mpi_sub_mpi( &Y, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &A ) == 0 );
 
 exit:
@@ -925,17 +968,20 @@
 
     res = mbedtls_mpi_sub_abs( &Z, &X, &Y );
     TEST_ASSERT( res == sub_result );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     if( res == 0 )
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
     /* result == first operand */
     TEST_ASSERT( mbedtls_mpi_sub_abs( &X, &X, &Y ) == sub_result );
+    TEST_ASSERT( sign_is_valid( &X ) );
     if( sub_result == 0 )
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
 
     /* result == second operand */
     TEST_ASSERT( mbedtls_mpi_sub_abs( &Y, &X, &Y ) == sub_result );
+    TEST_ASSERT( sign_is_valid( &Y ) );
     if( sub_result == 0 )
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Y, &A ) == 0 );
 
@@ -954,6 +1000,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_sub_int( &Z, &X, input_Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
 exit:
@@ -972,6 +1019,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &Y, radix_Y, input_Y ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_mul_mpi( &Z, &X, &Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
 
 exit:
@@ -990,6 +1038,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_mul_int( &Z, &X, input_Y ) == 0 );
+    TEST_ASSERT( sign_is_valid( &Z ) );
     if( strcmp( result_comparison, "==" ) == 0 )
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
     else if( strcmp( result_comparison, "!=" ) == 0 )
@@ -1020,6 +1069,8 @@
     TEST_ASSERT( res == div_result );
     if( res == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &Q ) );
+        TEST_ASSERT( sign_is_valid( &R ) );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Q, &A ) == 0 );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &R, &B ) == 0 );
     }
@@ -1047,6 +1098,8 @@
     TEST_ASSERT( res == div_result );
     if( res == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &Q ) );
+        TEST_ASSERT( sign_is_valid( &R ) );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Q, &A ) == 0 );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &R, &B ) == 0 );
     }
@@ -1073,6 +1126,7 @@
     TEST_ASSERT( res == div_result );
     if( res == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &X ) );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
     }
 
@@ -1126,6 +1180,7 @@
     TEST_ASSERT( res == div_result );
     if( res == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &Z ) );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &X ) == 0 );
     }
 
@@ -1185,6 +1240,7 @@
     TEST_ASSERT( res == div_result );
     if( res == 0 )
     {
+        TEST_ASSERT( sign_is_valid( &Z ) );
         TEST_ASSERT( mbedtls_mpi_cmp_mpi( &Z, &A ) == 0 );
     }
 
@@ -1259,6 +1315,7 @@
 
         TEST_ASSERT( actual_bits >= (size_t) bits );
         TEST_ASSERT( actual_bits <= (size_t) bits + 1 );
+        TEST_ASSERT( sign_is_valid( &X ) );
 
         TEST_ASSERT( mbedtls_mpi_is_prime_ext( &X, 40, rnd_std_rand, NULL )
                      == 0 );
@@ -1286,6 +1343,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_shift_l( &X, shift_X ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
 
 exit:
@@ -1303,6 +1361,7 @@
     TEST_ASSERT( mbedtls_test_read_mpi( &X, radix_X, input_X ) == 0 );
     TEST_ASSERT( mbedtls_test_read_mpi( &A, radix_A, input_A ) == 0 );
     TEST_ASSERT( mbedtls_mpi_shift_r( &X, shift_X ) == 0 );
+    TEST_ASSERT( sign_is_valid( &X ) );
     TEST_ASSERT( mbedtls_mpi_cmp_mpi( &X, &A ) == 0 );
 
 exit:
@@ -1334,6 +1393,7 @@
         TEST_ASSERT( mbedtls_mpi_size( &X ) + leading_zeros ==
                      (size_t) wanted_bytes );
         TEST_ASSERT( (int) bytes_left == rng_bytes - wanted_bytes );
+        TEST_ASSERT( sign_is_valid( &X ) );
     }
 
 exit: