diff --git a/tests/suites/test_suite_gcm.function b/tests/suites/test_suite_gcm.function
index a6b0a4c119..c7942f81a7 100644
--- a/tests/suites/test_suite_gcm.function
+++ b/tests/suites/test_suite_gcm.function
@@ -16,15 +16,19 @@ static int check_multipart( mbedtls_gcm_context *ctx,
     int ok = 0;
     uint8_t *output = NULL;
     size_t n2 = input->len - n1;
+    size_t n1_add = n1 < add->len ? add->len - n1 : add->len;
+    size_t n2_add = add->len - n1_add;
     size_t olen;
 
     /* Sanity checks on the test data */
     TEST_ASSERT( n1 <= input->len );
+    TEST_ASSERT( n1_add <= add->len );
     TEST_EQUAL( input->len, expected_output->len );
 
     TEST_EQUAL( 0, mbedtls_gcm_starts( ctx, mode,
                                          iv->x, iv->len ) );
-    TEST_EQUAL( 0, mbedtls_gcm_update_ad( ctx, add->x, add->len ) );
+    TEST_EQUAL( 0, mbedtls_gcm_update_ad( ctx, add->x, n1_add ) );
+    TEST_EQUAL( 0, mbedtls_gcm_update_ad( ctx, add->x + n1_add, n2_add ) );
 
     /* Allocate a tight buffer for each update call. This way, if the function
      * tries to write beyond the advertised required buffer size, this will