diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index 8b5f97d38..425e93ad2 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -25,6 +25,9 @@ mpi_read_write_string:16:"-20":10:"-32":100:0:0
 Base test mpi_read_write_string #3 (Negative decimal)
 mpi_read_write_string:16:"-23":16:"-23":100:0:0
 
+Base test mpi_read_write_string #4 (Buffer just fits)
+mpi_read_write_string:16:"-4":4:"-10":4:0:0
+
 Test mpi_read_write_string #1 (Invalid character)
 mpi_read_write_string:10:"a28":0:"":100:MBEDTLS_ERR_MPI_INVALID_CHARACTER:0
 
diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index d1fa5a46c..f982385e1 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -294,6 +294,8 @@ void mpi_read_write_string( int radix_X, char * input_X, int radix_A,
 
     mbedtls_mpi_init( &X );
 
+    memset( str, '!', sizeof( str ) );
+
     TEST_ASSERT( mbedtls_mpi_read_string( &X, radix_X, input_X ) == result_read );
     if( result_read == 0 )
     {
@@ -301,6 +303,7 @@ void mpi_read_write_string( int radix_X, char * input_X, int radix_A,
         if( result_write == 0 )
         {
             TEST_ASSERT( strcasecmp( str, input_A ) == 0 );
+            TEST_ASSERT( str[len] == '!' );
         }
     }