diff --git a/tests/suites/test_suite_chachapoly.function b/tests/suites/test_suite_chachapoly.function
index bdea57b8f2..670160b2fd 100644
--- a/tests/suites/test_suite_chachapoly.function
+++ b/tests/suites/test_suite_chachapoly.function
@@ -84,6 +84,7 @@ void chachapoly_update_too_long()
     unsigned char input[65] = { 0 };
     unsigned char output[65] = { 0 };
     mbedtls_chachapoly_context ctx;
+    size_t i;
 
     mbedtls_chachapoly_init(&ctx);
 
@@ -93,8 +94,12 @@ void chachapoly_update_too_long()
 
     /* Force the next update to be too long without processing a huge buffer. */
     ctx.chacha20_ctx.state[12] = UINT32_MAX;
+    memset(output, 0x2a, sizeof(output));
     TEST_EQUAL(mbedtls_chachapoly_update(&ctx, sizeof(input), input, output),
                MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA);
+    for (i = 0; i < sizeof(output); i++) {
+        TEST_EQUAL(output[i], 0x2a);
+    }
 
 exit:
     mbedtls_chachapoly_free(&ctx);