diff --git a/HISTORY.rst b/HISTORY.rst
index 47e3fe17..622254cb 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -26,6 +26,7 @@ PlatformIO Core 4
 * Fixed an issue when Python 2 does not keep encoding when converting ".ino" (`issue #3393 <https://github.com/platformio/platformio-core/issues/3393>`_)
 * Fixed an issue when ``"libArchive": false`` in "library.json" does not work (`issue #3403 <https://github.com/platformio/platformio-core/issues/3403>`_)
 * Fixed an issue when not all commands in `compilation database "compile_commands.json" <https://docs.platformio.org/page/integration/compile_commands.html>`__ use absolute paths (`pull #3415 <https://github.com/platformio/platformio-core/pull/3415>`_)
+* Fixed an issue when unknown transport is used for `PIO Unit Testing <https://docs.platformio.org/page/plus/unit-testing.html>`__ engine (`issue #3422 <https://github.com/platformio/platformio-core/issues/3422>`_)
 
 4.2.1 (2020-02-17)
 ~~~~~~~~~~~~~~~~~~
diff --git a/docs b/docs
index 05a2a8ff..ebf066f7 160000
--- a/docs
+++ b/docs
@@ -1 +1 @@
-Subproject commit 05a2a8ff4c1bb4121151cf8b32f4f8f05a709be5
+Subproject commit ebf066f7911e9b39ead1d0278c2b530e27c3c8be
diff --git a/platformio/commands/test/processor.py b/platformio/commands/test/processor.py
index 64a7d3dd..5291b9f7 100644
--- a/platformio/commands/test/processor.py
+++ b/platformio/commands/test/processor.py
@@ -83,6 +83,7 @@ class TestProcessorBase(object):
         self._outputcpp_generated = False
 
     def get_transport(self):
+        transport = None
         if self.env_options.get("platform") == "native":
             transport = "native"
         elif "framework" in self.env_options:
@@ -91,7 +92,9 @@ class TestProcessorBase(object):
             transport = self.env_options["test_transport"]
         if transport not in TRANSPORT_OPTIONS:
             raise exception.PlatformioException(
-                "Unknown Unit Test transport `%s`" % transport
+                "Unknown Unit Test transport `%s`. Please check a documentation how "
+                "to create an own 'Test Transport':\n"
+                "- https://docs.platformio.org/page/plus/unit-testing.html" % transport
             )
         return transport.lower()