diff --git a/platformio/commands/test/processor.py b/platformio/commands/test/processor.py
index e1a141c4..085ffb2c 100644
--- a/platformio/commands/test/processor.py
+++ b/platformio/commands/test/processor.py
@@ -148,7 +148,11 @@ class TestProcessorBase(object):
             "",
             "$object",
             "",
+            "#ifdef __GNUC__",
+            "void output_start(unsigned int baudrate __attribute__((unused)))",
+            "#else",
             "void output_start(unsigned int baudrate)",
+            "#endif",
             "{",
             "    $begin;",
             "}",