diff --git a/api/server/src/main/java/org/softwareheritage/graph/App.java b/api/server/src/main/java/org/softwareheritage/graph/App.java
index f54cc39..31ad1c4 100644
--- a/api/server/src/main/java/org/softwareheritage/graph/App.java
+++ b/api/server/src/main/java/org/softwareheritage/graph/App.java
@@ -1,62 +1,70 @@
 package org.softwareheritage.graph;
 
 import java.io.IOException;
-import java.util.Optional;
+import java.util.List;
+import java.util.Map;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.PropertyNamingStrategy;
 import io.javalin.Javalin;
 import io.javalin.json.JavalinJackson;
 
 import org.softwareheritage.graph.Graph;
 import org.softwareheritage.graph.SwhId;
 import org.softwareheritage.graph.algo.Stats;
 import org.softwareheritage.graph.algo.Visit;
 
 public class App {
   public static void main(String[] args) throws IOException {
     String path = args[0];
     Graph graph = new Graph(path);
     Stats stats = new Stats(path);
 
     // Clean up on exit
     Runtime.getRuntime().addShutdownHook(new Thread() {
       public void run() {
         try {
           graph.cleanUp();
         } catch (IOException e) {
           System.out.println("Could not clean up graph on exit: " + e);
         }
       }
     });
 
     // Configure Jackson JSON to use snake case naming style
     ObjectMapper objectMapper = JavalinJackson.getObjectMapper();
     objectMapper.setPropertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
     JavalinJackson.configure(objectMapper);
 
     Javalin app = Javalin.create().start(5010);
 
     app.get("/stats", ctx -> {
       ctx.json(stats);
     });
 
     app.get("/visit/:swh_id", ctx -> {
       try {
+        Map<String, List<String>> queryParamMap = ctx.queryParamMap();
+        for (String key : queryParamMap.keySet()) {
+          if (!key.matches("direction|edges|traversal")) {
+            throw new IllegalArgumentException("Unknown query string: " + key);
+          }
+        }
+
         SwhId swhId = new SwhId(ctx.pathParam("swh_id"));
 
         // By default, traversal is a forward DFS using all edges
-        String algorithm = Optional.ofNullable(ctx.queryParam("traversal")).orElse("dfs");
-        String direction = Optional.ofNullable(ctx.queryParam("direction")).orElse("forward");
-        String edges = Optional.ofNullable(ctx.queryParam("edges")).orElse("all");
+        String traversal = ctx.queryParam("traversal", "dfs");
+        String direction = ctx.queryParam("direction", "forward");
+        String edges = ctx.queryParam("edges", "all");
 
-        ctx.json(new Visit(graph, swhId, edges, algorithm, direction));
+        ctx.json(new Visit(graph, swhId, edges, traversal, direction));
       } catch (IllegalArgumentException e) {
         ctx.status(400);
         ctx.result(e.getMessage());
       }
     });
 
     app.error(404, ctx -> { ctx.result("Not found"); });
   }
 }