diff --git a/broker/JFJochBrokerHttp.cpp b/broker/JFJochBrokerHttp.cpp
index b417085ab..31ce0d92b 100644
--- a/broker/JFJochBrokerHttp.cpp
+++ b/broker/JFJochBrokerHttp.cpp
@@ -136,6 +136,20 @@ JFJochBrokerHttp::HttpError JFJochBrokerHttp::handleOperationException(const std
         throw;
     } catch (const WrongDAQStateException &) {
         return {500, error_message_json(ex.what(), "WrongDAQState"), "application/json"};
+    } catch (const JFJochException &e) {
+        // Settings that parse but do not fit what they are checked against (a module placement naming
+        // modules the detector does not have, a value out of range) are the request's fault, as a parsing
+        // failure is: 400, as the API documents it.
+        switch (e.Category()) {
+            case JFJochExceptionCategory::InputParameterInvalid:
+            case JFJochExceptionCategory::InputParameterAboveMax:
+            case JFJochExceptionCategory::InputParameterBelowMin:
+            case JFJochExceptionCategory::WrongNumber:
+            case JFJochExceptionCategory::WrongUnit:
+                return {400, e.what(), "text/plain"};
+            default:
+                return {500, error_message_json(ex.what(), "Other"), "application/json"};
+        }
     } catch (const std::exception &) {
         return {500, error_message_json(ex.what(), "Other"), "application/json"};
     }
diff --git a/tests/BrokerHttpAuthTest.cpp b/tests/BrokerHttpAuthTest.cpp
index 433ae10e0..20868cbe7 100644
--- a/tests/BrokerHttpAuthTest.cpp
+++ b/tests/BrokerHttpAuthTest.cpp
@@ -8,6 +8,7 @@
 #include <thread>
 
 #include "../broker/JFJochBrokerHttp.h"
+#include "../common/DetectorModules.h"
 
 // The tokens given to /start guard the endpoints that expose the dataset, and nothing else, for as
 // long as that dataset is the current one. Runs the real HTTP layer against a broker with no
@@ -159,3 +160,62 @@ TEST_CASE("JFJochBrokerHttp_BearerTokens", "[broker]") {
     }
 
 }
+
+// A module placement that does not name exactly the detector's modules is the request's fault: /start
+// answers 400, whether a module is left out, placed twice or not the detector's at all.
+TEST_CASE("JFJochBrokerHttp_StartModulePlacementMismatch", "[broker]") {
+    DiffractionExperiment experiment;
+    SpotFindingSettings spot_finding;
+    JFJochBrokerHttp broker(experiment, spot_finding);
+    broker.AddDetectorSetup(DetJF4M());
+
+    httplib::Server server;
+    broker.attach(server);
+    const int port = server.bind_to_any_port("127.0.0.1");
+    REQUIRE(port > 0);
+    struct ServerThread {
+        httplib::Server &server;
+        std::thread thread;
+        explicit ServerThread(httplib::Server &s) : server(s), thread([&s] { s.listen_after_bind(); }) {}
+        ~ServerThread() { server.stop(); thread.join(); }
+    } server_thread(server);
+    server.wait_until_ready();
+
+    httplib::Client client("127.0.0.1", port);
+    REQUIRE(client.Post("/initialize")->status == 200);
+
+    const auto assembled = ModulesAsImaged(DetJF4M().GetGeometry()).Get();
+    REQUIRE(assembled.size() == 8);
+    const auto entry = [](const DetectorModule &m) {
+        return nlohmann::json{{"box_x", m.x0}, {"box_y", m.y0}, {"box_width", m.width}, {"box_height", m.height},
+                              {"plane_x", m.ox}, {"plane_y", m.oy}, {"x_step_x", 1.0}, {"x_step_y", 0.0},
+                              {"y_step_x", 0.0}, {"y_step_y", 1.0}};
+    };
+    const auto start = [&](const nlohmann::json &placement) {
+        nlohmann::json body = {{"beam_x_pxl", 1000}, {"beam_y_pxl", 1000},
+                               {"detector_distance_mm", 100}, {"incident_energy_keV", 12.4},
+                               {"images_per_trigger", 1}, {"ntrigger", 1},
+                               {"file_prefix", "placement_run"}, {"module_placement", placement}};
+        auto res = client.Post("/start", body.dump(), "application/json");
+        REQUIRE(res);
+        return res->status;
+    };
+
+    nlohmann::json missing = nlohmann::json::array();
+    for (size_t m = 1; m < assembled.size(); m++)
+        missing.push_back(entry(assembled[m]));
+    CHECK(start(missing) == 400);
+
+    nlohmann::json twice = missing;
+    twice.push_back(entry(assembled[1]));
+    CHECK(start(twice) == 400);
+
+    DetectorModule elsewhere = assembled[0];
+    elsewhere.y0 += 1;
+    nlohmann::json unknown = missing;
+    unknown.push_back(entry(elsewhere));
+    CHECK(start(unknown) == 400);
+
+    // Nothing was started.
+    CHECK(nlohmann::json::parse(client.Get("/status")->body).at("state") == "Idle");
+}