diff --git a/client/src/api/schema/schema.ts b/client/src/api/schema/schema.ts
index 91c22b3eb39..1abd1423293 100644
--- a/client/src/api/schema/schema.ts
+++ b/client/src/api/schema/schema.ts
@@ -17056,7 +17056,9 @@ export interface components {
              * @default {}
              */
             parameters: {
-                [key: string]: unknown;
+                [key: string]: {
+                    [key: string]: unknown;
+                };
             } | null;
             /**
              * Legacy Step Parameters Normalized
diff --git a/lib/galaxy/schema/workflows.py b/lib/galaxy/schema/workflows.py
index 0a7399e27b8..0411d554b6e 100644
--- a/lib/galaxy/schema/workflows.py
+++ b/lib/galaxy/schema/workflows.py
@@ -178,7 +178,7 @@ class InvokeWorkflowPayload(GetTargetHistoryPayload):
             return json.loads(v)
         return v
 
-    parameters: Optional[dict[str, Any]] = Field(
+    parameters: Optional[dict[str, dict[str, Any]]] = Field(
         {},
         title=STEP_PARAMETERS_TITLE,
         description=STEP_PARAMETERS_DESCRIPTION,
diff --git a/lib/galaxy_test/api/test_workflows.py b/lib/galaxy_test/api/test_workflows.py
index 89417a8f55a..55dbba99934 100644
--- a/lib/galaxy_test/api/test_workflows.py
+++ b/lib/galaxy_test/api/test_workflows.py
@@ -28,6 +28,7 @@ from galaxy.exceptions import error_codes
 from galaxy.tool_util_models import UserToolSource
 from galaxy.util import UNKNOWN
 from galaxy_test.base import rules_test_data
+from galaxy_test.base.api_asserts import assert_error_message_contains
 from galaxy_test.base.populators import (
     DatasetCollectionPopulator,
     DatasetPopulator,
@@ -8231,6 +8232,14 @@ steps:
         self.__assert_lines_hid_line_count_is(history_id, 2, 5)
         self.__assert_lines_hid_line_count_is(history_id, 3, 5)
 
+    @skip_without_tool("random_lines1")
+    def test_run_replace_params_by_tool_rejects_scalar_values(self):
+        workflow_request, history_id, workflow_id = self._setup_random_x2_workflow("test_for_reject_scalar_params")
+        workflow_request["parameters"] = dumps(dict(random_lines1=5))
+        response = self.workflow_populator.invoke_workflow_raw(workflow_id, workflow_request)
+        self._assert_status_code_is(response, 400)
+        assert_error_message_contains(response, "Input should be a valid dictionary")
+
     @skip_without_tool("random_lines1")
     def test_run_replace_params_by_uuid(self):
         workflow_request, history_id, workflow_id = self._setup_random_x2_workflow("test_for_replace_")