diff --git a/cylc/flow/command_validation.py b/cylc/flow/command_validation.py
index e5f87e85ae6..d87c0711a8d 100644
--- a/cylc/flow/command_validation.py
+++ b/cylc/flow/command_validation.py
@@ -30,7 +30,7 @@
 
 
 ERR_OPT_FLOW_VAL = "Flow values must be an integer, or 'all', 'new', or 'none'"
-ERR_OPT_FLOW_INT = "Multiple flow options must all be integer valued"
+ERR_OPT_FLOW_COMBINE = "Cannot combine --flow={0} with other flow values"
 ERR_OPT_FLOW_WAIT = (
     f"--wait is not compatible with --flow={FLOW_NEW} or --flow={FLOW_NONE}"
 )
@@ -51,7 +51,8 @@ def flow_opts(flows: List[str], flow_wait: bool) -> None:
         Bad:
         >>> flow_opts(["none", "1"], False)
         Traceback (most recent call last):
-        cylc.flow.exceptions.InputError: ... must all be integer valued
+        cylc.flow.exceptions.InputError: Cannot combine --flow=none with other
+        flow values
 
         >>> flow_opts(["cheese", "2"], True)
         Traceback (most recent call last):
@@ -59,24 +60,26 @@ def flow_opts(flows: List[str], flow_wait: bool) -> None:
 
         >>> flow_opts(["new"], True)
         Traceback (most recent call last):
-        cylc.flow.exceptions.InputError: ...
+        cylc.flow.exceptions.InputError: --wait is not compatible with
+        --flow=new or --flow=none
 
     """
     if not flows:
         return
 
+    flows = [val.strip() for val in flows]
+
     for val in flows:
-        val = val.strip()
-        if val in [FLOW_NONE, FLOW_NEW, FLOW_ALL]:
+        if val in {FLOW_NONE, FLOW_NEW, FLOW_ALL}:
             if len(flows) != 1:
-                raise InputError(ERR_OPT_FLOW_INT)
+                raise InputError(ERR_OPT_FLOW_COMBINE.format(val))
         else:
             try:
                 int(val)
             except ValueError:
-                raise InputError(ERR_OPT_FLOW_VAL.format(val))
+                raise InputError(ERR_OPT_FLOW_VAL)
 
-    if flow_wait and flows[0] in [FLOW_NEW, FLOW_NONE]:
+    if flow_wait and flows[0] in {FLOW_NEW, FLOW_NONE}:
         raise InputError(ERR_OPT_FLOW_WAIT)
 
 
diff --git a/tests/integration/test_trigger.py b/tests/integration/test_trigger.py
index 3f6b5dee138..d9c5304b745 100644
--- a/tests/integration/test_trigger.py
+++ b/tests/integration/test_trigger.py
@@ -14,33 +14,9 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import logging
-
-from cylc.flow.flow_mgr import FLOW_ALL, FLOW_NEW, FLOW_NONE
-from cylc.flow.command_validation import flow_opts
-from cylc.flow.exceptions import InputError
-
-import pytest
 import time
 
-
-@pytest.mark.parametrize(
-    'flow_strs',
-    (
-        [FLOW_ALL, '1'],
-        ['1', FLOW_ALL],
-        [FLOW_NEW, '1'],
-        [FLOW_NONE, '1'],
-        ['a'],
-        ['1', 'a'],
-    )
-)
-async def test_trigger_invalid(mod_one, start, log_filter, flow_strs):
-    """Ensure invalid flow values are rejected during command validation."""
-    async with start(mod_one) as log:
-        log.clear()
-        with pytest.raises(InputError):
-            flow_opts(flow_strs, False)
+from cylc.flow.flow_mgr import FLOW_ALL
 
 
 async def test_trigger_no_flows(one, start, log_filter):
diff --git a/tests/unit/test_command_validation.py b/tests/unit/test_command_validation.py
new file mode 100644
index 00000000000..42fdda5aedf
--- /dev/null
+++ b/tests/unit/test_command_validation.py
@@ -0,0 +1,41 @@
+# THIS FILE IS PART OF THE CYLC WORKFLOW ENGINE.
+# Copyright (C) NIWA & British Crown (Met Office) & Contributors.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import pytest
+
+from cylc.flow.command_validation import (
+    ERR_OPT_FLOW_COMBINE,
+    ERR_OPT_FLOW_VAL,
+    flow_opts,
+)
+from cylc.flow.exceptions import InputError
+from cylc.flow.flow_mgr import FLOW_ALL, FLOW_NEW, FLOW_NONE
+
+
+@pytest.mark.parametrize('flow_strs, expected_msg', [
+    ([FLOW_ALL, '1'], ERR_OPT_FLOW_COMBINE.format(FLOW_ALL)),
+    (['1', FLOW_ALL], ERR_OPT_FLOW_COMBINE.format(FLOW_ALL)),
+    ([FLOW_NEW, '1'], ERR_OPT_FLOW_COMBINE.format(FLOW_NEW)),
+    ([FLOW_NONE, '1'], ERR_OPT_FLOW_COMBINE.format(FLOW_NONE)),
+    ([FLOW_NONE, FLOW_ALL], ERR_OPT_FLOW_COMBINE.format(FLOW_NONE)),
+    (['a'], ERR_OPT_FLOW_VAL),
+    (['1', 'a'], ERR_OPT_FLOW_VAL),
+])
+async def test_trigger_invalid(flow_strs, expected_msg):
+    """Ensure invalid flow values are rejected during command validation."""
+    with pytest.raises(InputError) as exc_info:
+        flow_opts(flow_strs, False)
+    assert str(exc_info.value) == expected_msg