diff --git a/cylc/rose/platform_utils.py b/cylc/rose/platform_utils.py
index 86eeac98..1994b535 100644
--- a/cylc/rose/platform_utils.py
+++ b/cylc/rose/platform_utils.py
@@ -22,6 +22,7 @@
 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
@@ -47,6 +48,11 @@ def get_platform_from_task_def(
     # 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 because the platform definition for'
+            f' task {task} is {task_spec["platform"]}.'
+        )
     return platform
 
 
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',
     [