diff --git a/src/engine/api/json_factory.cpp b/src/engine/api/json_factory.cpp
index 7d0dfdf83..e6b6b4c83 100644
--- a/src/engine/api/json_factory.cpp
+++ b/src/engine/api/json_factory.cpp
@@ -50,7 +50,7 @@ const constexpr char *turn_type_names[] = {
     "roundabout",      "roundabout", "rotary",   "rotary",      "roundabout turn",
     "roundabout turn", "use lane",   "invalid",  "invalid",     "invalid",
     "invalid",         "invalid",    "invalid",  "invalid",     "invalid",
-    "invalid"};
+    "invalid",         "invalid"};
 
 const constexpr char *waypoint_type_names[] = {"invalid", "arrive", "depart"};
 
@@ -68,6 +68,7 @@ inline bool hasValidLanes(const guidance::Intersection &intersection)
 
 std::string instructionTypeToString(const TurnType::Enum type)
 {
+    BOOST_ASSERT(static_cast<std::size_t>(type) < sizeof(turn_type_names)/sizeof(turn_type_names[0]));
     return turn_type_names[static_cast<std::size_t>(type)];
 }
 
diff --git a/unit_tests/engine/json_factory.cpp b/unit_tests/engine/json_factory.cpp
new file mode 100644
index 000000000..a246af072
--- /dev/null
+++ b/unit_tests/engine/json_factory.cpp
@@ -0,0 +1,16 @@
+#include "engine/api/json_factory.hpp"
+
+#include <boost/test/test_case_template.hpp>
+#include <boost/test/unit_test.hpp>
+
+BOOST_AUTO_TEST_SUITE(json_factory)
+
+BOOST_AUTO_TEST_CASE(instructionTypeToString_test_size)
+{
+    using namespace osrm::engine::api::json::detail;
+    using namespace osrm::extractor::guidance;
+
+    BOOST_CHECK_EQUAL(instructionTypeToString(TurnType::Sliproad), "invalid");
+}
+
+BOOST_AUTO_TEST_SUITE_END()