diff --git a/tests/suites/test_suite_mpi.function b/tests/suites/test_suite_mpi.function
index 5358379f0..67894e654 100644
--- a/tests/suites/test_suite_mpi.function
+++ b/tests/suites/test_suite_mpi.function
@@ -313,14 +313,14 @@ exit:
 void mbedtls_mpi_read_binary( data_t * buf, int radix_A, char * input_A )
 {
     mbedtls_mpi X;
-    unsigned char str[1000];
+    char str[1000];
     size_t len;
 
     mbedtls_mpi_init( &X );
 
 
     TEST_ASSERT( mbedtls_mpi_read_binary( &X, buf->x, buf->len ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, (char *) str, sizeof( str ), &len ) == 0 );
+    TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, str, sizeof( str ), &len ) == 0 );
     TEST_ASSERT( strcmp( (char *) str, input_A ) == 0 );
 
 exit:
@@ -332,14 +332,14 @@ exit:
 void mbedtls_mpi_read_binary_le( data_t * buf, int radix_A, char * input_A )
 {
     mbedtls_mpi X;
-    unsigned char str[1000];
+    char str[1000];
     size_t len;
 
     mbedtls_mpi_init( &X );
 
 
     TEST_ASSERT( mbedtls_mpi_read_binary_le( &X, buf->x, buf->len ) == 0 );
-    TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, (char *) str, sizeof( str ), &len ) == 0 );
+    TEST_ASSERT( mbedtls_mpi_write_string( &X, radix_A, str, sizeof( str ), &len ) == 0 );
     TEST_ASSERT( strcmp( (char *) str, input_A ) == 0 );
 
 exit: