diff --git a/src/specify_cli/workflows/steps/if_then/__init__.py b/src/specify_cli/workflows/steps/if_then/__init__.py
index e7179a418a..9e775df0b9 100644
--- a/src/specify_cli/workflows/steps/if_then/__init__.py
+++ b/src/specify_cli/workflows/steps/if_then/__init__.py
@@ -23,8 +23,30 @@ def execute(self, config: dict[str, Any], context: StepContext) -> StepResult:
 
         if result:
             branch = config.get("then", [])
+            branch_name = "then"
         else:
             branch = config.get("else", [])
+            branch_name = "else"
+
+        if not isinstance(branch, list):
+            # The engine does not auto-validate step config and feeds
+            # ``next_steps`` straight into ``_execute_steps``, which iterates them
+            # as step mappings. A non-list ``then``/``else`` (a single mapping or
+            # scalar authoring mistake) would otherwise be iterated element-wise
+            # — a dict yields its keys, a str its characters — and crash the whole
+            # run with AttributeError on ``.get()``. ``validate`` already rejects
+            # a non-list branch; fail this step loudly on an unvalidated run
+            # instead, mirroring the while/do-while (#3519) / switch / fan-out
+            # steps. The selected branch is always returned as next_steps, so the
+            # guard is unconditional.
+            return StepResult(
+                status=StepStatus.FAILED,
+                error=(
+                    f"If step {config.get('id', '?')!r}: {branch_name!r} must be a "
+                    f"list of steps, got {type(branch).__name__}."
+                ),
+                output={"condition_result": result},
+            )
 
         return StepResult(
             status=StepStatus.COMPLETED,
diff --git a/tests/test_workflows.py b/tests/test_workflows.py
index a766283119..57bb18cba1 100644
--- a/tests/test_workflows.py
+++ b/tests/test_workflows.py
@@ -2060,6 +2060,30 @@ def test_execute_else_branch(self):
         assert result.output["condition_result"] is False
         assert result.next_steps[0]["id"] == "b"
 
+    @pytest.mark.parametrize("bad_branch", ["oops", {"a": 1}, 42])
+    def test_execute_rejects_non_list_branch(self, bad_branch):
+        """execute() fails cleanly on a non-list then/else branch. The engine
+        doesn't auto-validate step config, so a non-iterable next_steps would
+        otherwise crash the run — completing the while/do-while guard (#3519) for
+        the if step (mirrors the switch step's 'cases' guard)."""
+        from specify_cli.workflows.steps.if_then import IfThenStep
+        from specify_cli.workflows.base import StepContext, StepStatus
+
+        step = IfThenStep()
+        cond = "{{ inputs.scope == 'full' }}"
+        res_then = step.execute(
+            {"id": "i", "condition": cond, "then": bad_branch},
+            StepContext(inputs={"scope": "full"}),
+        )
+        assert res_then.status is StepStatus.FAILED
+        assert "'then' must be a list" in (res_then.error or "")
+        res_else = step.execute(
+            {"id": "i", "condition": cond, "then": [], "else": bad_branch},
+            StepContext(inputs={"scope": "backend"}),
+        )
+        assert res_else.status is StepStatus.FAILED
+        assert "'else' must be a list" in (res_else.error or "")
+
     def test_validate_missing_condition(self):
         from specify_cli.workflows.steps.if_then import IfThenStep