diff --git a/src/geom-from-osm.cpp b/src/geom-from-osm.cpp
index 339148300..968f52d25 100644
--- a/src/geom-from-osm.cpp
+++ b/src/geom-from-osm.cpp
@@ -85,13 +85,15 @@ void fill_polygon(polygon_t *polygon, osmium::Area const &area,
 
 } // anonymous namespace
 
-void create_linestring(geometry_t *geom, osmium::Way const &way)
+bool create_linestring(geometry_t *geom, osmium::Way const &way)
 {
     auto &line = geom->set<linestring_t>();
 
     if (!fill_point_list(&line, way.nodes())) {
         geom->reset();
     }
+
+    return way.nodes().size() == line.size();
 }
 
 geometry_t create_linestring(osmium::Way const &way)
@@ -101,7 +103,7 @@ geometry_t create_linestring(osmium::Way const &way)
     return geom;
 }
 
-void create_polygon(geometry_t *geom, osmium::Way const &way,
+bool create_polygon(geometry_t *geom, osmium::Way const &way,
                     osmium::memory::Buffer *area_buffer)
 {
     auto &polygon = geom->set<polygon_t>();
@@ -109,20 +111,22 @@ void create_polygon(geometry_t *geom, osmium::Way const &way,
     // A closed way with less than 4 nodes can never be a valid polygon
     if (way.nodes().size() < 4U) {
         geom->reset();
-        return;
+        return false;
     }
 
     geom::area_assembler_t assembler{area_buffer};
 
     if (!assembler(way)) {
         geom->reset();
-        return;
+        return false;
     }
 
     auto const &area = assembler.get_area();
     auto const &ring = *area.cbegin<osmium::OuterRing>();
 
     fill_point_list(&polygon.outer(), ring);
+
+    return assembler.stats().duplicate_nodes == 0;
 }
 
 geometry_t create_polygon(osmium::Way const &way,
diff --git a/src/geom-from-osm.hpp b/src/geom-from-osm.hpp
index a1068e87d..65fe6abe4 100644
--- a/src/geom-from-osm.hpp
+++ b/src/geom-from-osm.hpp
@@ -63,7 +63,7 @@ void create_point(geometry_t *geom, osmium::Node const &node);
  * \param geom Pointer to an existing geometry which will be used as output.
  * \param way The input way.
  */
-void create_linestring(geometry_t *geom, osmium::Way const &way);
+bool create_linestring(geometry_t *geom, osmium::Way const &way);
 
 /**
  * Create a linestring geometry from a way. Nodes without location are ignored.
@@ -87,7 +87,7 @@ void create_linestring(geometry_t *geom, osmium::Way const &way);
  * \param way The input way.
  * \param area_buffer Temporary buffer used to create area.
  */
-void create_polygon(geometry_t *geom, osmium::Way const &way,
+bool create_polygon(geometry_t *geom, osmium::Way const &way,
                     osmium::memory::Buffer *area_buffer);
 
 /**
diff --git a/src/output-flex.cpp b/src/output-flex.cpp
index b8a41fbed..840110544 100644
--- a/src/output-flex.cpp
+++ b/src/output-flex.cpp
@@ -504,9 +504,14 @@ int output_flex_t::app_as_linestring()
     m_way_cache.add_nodes(middle());
 
     auto *geom = create_lua_geometry_object(lua_state());
-    geom::create_linestring(geom, m_way_cache.get());
+    bool const okay = geom::create_linestring(geom, m_way_cache.get());
 
-    return 1;
+    if (geom->is_null()) {
+        return 1;
+    }
+
+    lua_pushboolean(lua_state(), okay);
+    return 2;
 }
 
 int output_flex_t::app_as_polygon()
@@ -517,9 +522,15 @@ int output_flex_t::app_as_polygon()
     m_way_cache.add_nodes(middle());
 
     auto *geom = create_lua_geometry_object(lua_state());
-    geom::create_polygon(geom, m_way_cache.get(), &m_area_buffer);
+    bool const okay =
+        geom::create_polygon(geom, m_way_cache.get(), &m_area_buffer);
 
-    return 1;
+    if (geom->is_null()) {
+        return 1;
+    }
+
+    lua_pushboolean(lua_state(), okay);
+    return 2;
 }
 
 int output_flex_t::app_as_multipoint()
diff --git a/tests/bdd/flex/geometry-linestring.feature b/tests/bdd/flex/geometry-linestring.feature
index 6bd2f0d5e..1397aad57 100644
--- a/tests/bdd/flex/geometry-linestring.feature
+++ b/tests/bdd/flex/geometry-linestring.feature
@@ -66,3 +66,46 @@ Feature: Creating linestring features from way
             Geometry data for geometry column 'geom' has the wrong type (LINESTRING).
             """
 
+    Scenario:
+        Given the grid
+            | 1 | 2 |   |
+            |   |   | 3 |
+        And the OSM data
+            """
+            w20 Thighway=motorway Nn1,n2,n3,n1
+            w21 Thighway=motorway Nn1,n2,n2,n3,n1
+            w22 Thighway=motorway Nn1,n2,n3,n1,n1
+            w23 Thighway=motorway Nn1,n2,n3
+            w24 Thighway=motorway Nn2,n2
+            """
+        And the lua style
+            """
+            local lines = osm2pgsql.define_way_table('osm2pgsql_test_lines', {
+                { column = 'lgeom', type = 'linestring', projection = 4326 },
+                { column = 'pgeom', type = 'polygon', projection = 4326 },
+                { column = 'lclean', type = 'bool' },
+                { column = 'pclean', type = 'bool' },
+            })
+
+            function osm2pgsql.process_way(object)
+                lgeom, lclean = object:as_linestring()
+                pgeom, pclean = object:as_polygon()
+                lines:insert({
+                    lgeom = lgeom,
+                    pgeom = pgeom,
+                    lclean = lclean,
+                    pclean = pclean,
+                })
+            end
+
+            """
+        When running osm2pgsql flex
+
+        Then table osm2pgsql_test_lines contains exactly
+            | way_id | lgeom!geo  | lclean | pgeom!geo    | pclean |
+            | 20     | 1, 2, 3, 1 | True   | (1, 2, 3, 1) | True   |
+            | 21     | 1, 2, 3, 1 | False  | (1, 2, 3, 1) | False  |
+            | 22     | 1, 2, 3, 1 | False  | (1, 2, 3, 1) | False  |
+            | 23     | 1, 2, 3    | True   | NULL         | NULL   |
+            | 24     | NULL       | NULL   | NULL         | NULL   |
+