diff --git a/tools/testing/selftests/tc-testing/README b/tools/testing/selftests/tc-testing/README
index 22e5da9008fd..b0954c873e2f 100644
--- a/tools/testing/selftests/tc-testing/README
+++ b/tools/testing/selftests/tc-testing/README
@@ -128,7 +128,9 @@ optional arguments:
   -v, --verbose         Show the commands that are being run
   -N, --notap           Suppress tap results for command under test
   -d DEVICE, --device DEVICE
-                        Execute the test case in flower category
+                        Execute test cases that use a physical device, where
+                        DEVICE is its name. (If not defined, tests that require
+                        a physical device will be skipped)
   -P, --pause           Pause execution just before post-suite stage
 
 selection:
diff --git a/tools/testing/selftests/tc-testing/tdc.py b/tools/testing/selftests/tc-testing/tdc.py
index f04321ace9fb..e566c70e64a1 100755
--- a/tools/testing/selftests/tc-testing/tdc.py
+++ b/tools/testing/selftests/tc-testing/tdc.py
@@ -356,12 +356,14 @@ def test_runner(pm, args, filtered_tests):
     time.sleep(2)
     for tidx in testlist:
         if "flower" in tidx["category"] and args.device == None:
+            errmsg = "Tests using the DEV2 variable must define the name of a "
+            errmsg += "physical NIC with the -d option when running tdc.\n"
+            errmsg += "Test has been skipped."
             if args.verbose > 1:
-                print('Not executing test {} {} because DEV2 not defined'.
-                      format(tidx['id'], tidx['name']))
+                print(errmsg)
             res = TestResult(tidx['id'], tidx['name'])
             res.set_result(ResultState.skip)
-            res.set_errormsg('Not executed because DEV2 is not defined')
+            res.set_errormsg(errmsg)
             tsr.add_resultdata(res)
             continue
         try:
@@ -499,7 +501,9 @@ def set_args(parser):
         choices=['none', 'xunit', 'tap'],
         help='Specify the format for test results. (Default: TAP)')
     parser.add_argument('-d', '--device',
-                        help='Execute the test case in flower category')
+                        help='Execute test cases that use a physical device, ' +
+                        'where DEVICE is its name. (If not defined, tests ' +
+                        'that require a physical device will be skipped)')
     parser.add_argument(
         '-P', '--pause', action='store_true',
         help='Pause execution just before post-suite stage')