diff --git a/src/specify_cli/workflows/steps/fan_in/__init__.py b/src/specify_cli/workflows/steps/fan_in/__init__.py
index dec3e3fd4..7b82df50b 100644
--- a/src/specify_cli/workflows/steps/fan_in/__init__.py
+++ b/src/specify_cli/workflows/steps/fan_in/__init__.py
@@ -58,4 +58,13 @@ class FanInStep(StepBase):
                 f"Fan-in step {config.get('id', '?')!r}: "
                 f"'wait_for' must be a non-empty list of step IDs."
             )
+        output = config.get("output")
+        if output is not None and not isinstance(output, dict):
+            # execute() silently coerces a non-mapping output to {}, so the
+            # author's declared aggregation keys would vanish with no error.
+            # Reject at validation, mirroring the command-step (#3262) fix.
+            errors.append(
+                f"Fan-in step {config.get('id', '?')!r}: 'output' must be a "
+                f"mapping of key -> expression, got {type(output).__name__}."
+            )
         return errors
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index 77868cb34..c8e6b5bed 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -2202,6 +2202,27 @@ class TestFanInStep:
         errors = step.validate({"id": "test", "wait_for": "not-a-list"})
         assert any("non-empty list" in e for e in errors)
 
+    @pytest.mark.parametrize("bad_output", [["{{ fan_in.results }}"], "{{ x }}", 42])
+    def test_validate_rejects_non_mapping_output(self, bad_output):
+        """A non-mapping 'output' must be rejected: execute() would otherwise
+        silently coerce it to {} and drop the declared aggregation keys."""
+        from specify_cli.workflows.steps.fan_in import FanInStep
+
+        step = FanInStep()
+        errors = step.validate(
+            {"id": "j", "wait_for": ["a"], "output": bad_output}
+        )
+        assert any("'output' must be a mapping" in e for e in errors)
+
+    def test_validate_accepts_mapping_or_absent_output(self):
+        from specify_cli.workflows.steps.fan_in import FanInStep
+
+        step = FanInStep()
+        assert step.validate(
+            {"id": "j", "wait_for": ["a"], "output": {"joined": "{{ x }}"}}
+        ) == []
+        assert step.validate({"id": "j", "wait_for": ["a"]}) == []
+
 
 class TestFanOutConcurrency:
     """Fan-out honors max_concurrency (WorkflowEngine._run_fan_out)."""