diff --git a/src/gen/osm2pgsql-gen.cpp b/src/gen/osm2pgsql-gen.cpp
index 31cbaa50b..55a9ce46e 100644
--- a/src/gen/osm2pgsql-gen.cpp
+++ b/src/gen/osm2pgsql-gen.cpp
@@ -796,6 +796,11 @@ int main(int argc, char *argv[])
             }
         }
 
+        if (properties.get_string("output", "flex") != "flex") {
+            throw std::runtime_error{
+                "osm2pgsql-gen only works with flex output"};
+        }
+
         bool const updatable = properties.get_bool("updatable", false);
         genproc_t gen{style, conninfo, dbschema, append, updatable, jobs};
         gen.run();