diff --git a/api/QuestionsApi/Controllers/ExamSimulationsController.cs b/api/QuestionsApi/Controllers/ExamSimulationsController.cs
index 8e93819..6752192 100644
--- a/api/QuestionsApi/Controllers/ExamSimulationsController.cs
+++ b/api/QuestionsApi/Controllers/ExamSimulationsController.cs
@@ -41,8 +41,15 @@ public class ExamSimulationsController(ExamSimulationStore simulations) : Contro
     }
 
     [HttpGet("completed")]
-    public async Task<IActionResult> Completed([FromQuery] int limit = 20, [FromQuery] DateTime? before = null) =>
-        Ok(await simulations.GetCompletedAsync(UserId(), limit, before));
+    public async Task<IActionResult> Completed([FromQuery] int limit = 20, [FromQuery] string? before = null)
+    {
+        DateTime? cursor = null;
+        if (!string.IsNullOrWhiteSpace(before) && !DateTime.TryParse(before, null, System.Globalization.DateTimeStyles.RoundtripKind, out var parsed))
+            return BadRequest("The completed-simulation cursor is invalid.");
+        else if (!string.IsNullOrWhiteSpace(before))
+            cursor = parsed;
+        return Ok(await simulations.GetCompletedAsync(UserId(), limit, cursor));
+    }
 
     [HttpGet("{id}")]
     public async Task<IActionResult> Get(string id)
diff --git a/scripts/autoload/exam_simulation_service.gd b/scripts/autoload/exam_simulation_service.gd
index 467d1fa..d7c2e1c 100644
--- a/scripts/autoload/exam_simulation_service.gd
+++ b/scripts/autoload/exam_simulation_service.gd
@@ -173,7 +173,8 @@ func _on_request_completed(result: int, response_code: int, _headers: PackedStri
 	if action == "history" or action == "history_append":
 		var items: Array = parsed.get("items", [])
 		completed_history = completed_history + items if action == "history_append" else items
-		completed_history_next_before = str(parsed.get("nextBefore", ""))
+		var next_before: Variant = parsed.get("nextBefore", null)
+		completed_history_next_before = "" if next_before == null else str(next_before)
 		completed_history_changed.emit()
 		return
 	if action.begins_with("outcome_"):