diff --git a/cylc/rose/platform_utils.py b/cylc/rose/platform_utils.py
index c44c33b4..eef57631 100644
--- a/cylc/rose/platform_utils.py
+++ b/cylc/rose/platform_utils.py
@@ -19,9 +19,10 @@
 """
 from optparse import Values
 from pathlib import Path
-from typing import Optional, Dict, Any
+from typing import Dict, Any
 
 from cylc.flow.config import WorkflowConfig
+from cylc.flow.exceptions import PlatformLookupError
 from cylc.flow.rundb import CylcWorkflowDAO
 from cylc.flow.workflow_files import parse_reg
 from cylc.flow.platforms import get_platform
@@ -29,7 +30,7 @@
 
 def get_platform_from_task_def(
     flow: str, task: str
-) -> Optional[Dict[str, Any]]:
+) -> Dict[str, Any]:
     """Return the platform dictionary for a particular task.
 
     Uses the flow definition - designed to be used with tasks
@@ -42,17 +43,22 @@ def get_platform_from_task_def(
     Returns:
         Platform Dictionary.
     """
-    flow_name, flow_file = parse_reg(flow, src=True)
+    _, flow_file = parse_reg(flow, src=True)
     config = WorkflowConfig(flow, flow_file, Values())
     # Get entire task spec to allow Cylc 7 platform from host guessing.
     task_spec = config.pcfg.get(['runtime', task])
     platform = get_platform(task_spec)
+    if platform is None:
+        raise PlatformLookupError(
+            'Platform lookup failed; platform is a subshell to be evaluated: '
+            f' Task: {task}, platform: {task_spec["platform"]}.'
+        )
     return platform
 
 
 def get_platforms_from_task_jobs(
     flow: str, cyclepoint: str
-) -> Optional[Dict[str, Any]]:
+) -> Dict[str, Any]:
     """Access flow database. Return platform for task at fixed cycle point
 
     Uses the workflow database - designed to be used with tasks where jobs
@@ -66,7 +72,7 @@ def get_platforms_from_task_jobs(
     Returns:
         Platform Dictionary.
     """
-    flow_name, flow_file = parse_reg(flow, src=True)
+    _, flow_file = parse_reg(flow, src=True)
     dbfilepath = Path(flow_file).parent / '.service/db'
     dao = CylcWorkflowDAO(dbfilepath)
     task_platform_map: Dict = {}
diff --git a/tests/test_platform_utils.py b/tests/test_platform_utils.py
index fb6661c1..046c4d44 100644
--- a/tests/test_platform_utils.py
+++ b/tests/test_platform_utils.py
@@ -25,6 +25,7 @@
 from shutil import rmtree
 from subprocess import run
 from uuid import uuid4
+from cylc.flow.exceptions import PlatformLookupError
 
 from cylc.rose.platform_utils import (
     get_platform_from_task_def,
@@ -125,6 +126,8 @@ def fake_flow():
             [[qux]]
                 [[[remote]]]
                     host = cheese
+            [[kanga]]
+                platform = $(echo "myplatform")
             [[BAR]]
                 platform = milk
             [[child_of_bar]]
@@ -180,6 +183,20 @@ def test_get_platform_from_task_def(
     assert platform['name'] == expected_platform_n
 
 
+def test_get_platform_from_task_def_raises(
+    mock_glbl_cfg, fake_flow
+):
+    """Test getting platform from task definition.
+
+    This is approaching an integration test, because
+    although it's only testing one unit of Cylc Rose, that unit
+    is calling lots of Cylc Parts, which aren't mocked.
+    """
+    mock_glbl_cfg(*MOCK_GLBL_CFG)
+    with pytest.raises(PlatformLookupError, match='Platform lookup failed.*'):
+        get_platform_from_task_def(fake_flow[0], 'kanga')
+
+
 @pytest.mark.parametrize(
     'task, cycle, expect',
     [