diff --git a/serial.cpp b/serial.cpp
index 49d8703d..c3193b17 100644
--- a/serial.cpp
+++ b/serial.cpp
@@ -383,7 +383,7 @@ static void write_geometry(drawvec const &dv, std::string &out, long long wx, lo
 }
 
 // called from generating the next zoom level
-void serialize_feature(FILE *geomfile, serial_feature *sf, std::atomic<long long> *geompos, const char *fname, long long wx, long long wy) {
+std::string serialize_feature(serial_feature *sf, long long wx, long long wy) {
 	std::string s;
 
 	serialize_byte(s, sf->t);
@@ -445,9 +445,7 @@ void serialize_feature(FILE *geomfile, serial_feature *sf, std::atomic<long long
 
 	// MAGIC: This knows that the feature minzoom is the last byte of the feature,
 	serialize_byte(s, sf->feature_minzoom);
-
-	serialize_long_long(geomfile, s.size(), geompos, fname);
-	fwrite_check(s.c_str(), sizeof(char), s.size(), geomfile, geompos, fname);
+	return s;
 }
 
 serial_feature deserialize_feature(FILE *geoms, std::atomic<long long> *geompos_in, unsigned z, unsigned tx, unsigned ty, unsigned *initial_x, unsigned *initial_y) {
@@ -917,7 +915,10 @@ int serialize_feature(struct serialization_state *sst, serial_feature &sf) {
 
 	long long geomstart = r->geompos;
 	sf.geometry = scaled_geometry;
-	serialize_feature(r->geomfile, &sf, &r->geompos, sst->fname, SHIFT_RIGHT(*(sst->initial_x)), SHIFT_RIGHT(*(sst->initial_y)));
+
+	std::string feature = serialize_feature(&sf, SHIFT_RIGHT(*(sst->initial_x)), SHIFT_RIGHT(*(sst->initial_y)));
+	serialize_long_long(r->geomfile, feature.size(), &r->geompos, sst->fname);
+	fwrite_check(feature.c_str(), sizeof(char), feature.size(), r->geomfile, &r->geompos, sst->fname);
 
 	struct index index;
 	index.start = geomstart;
diff --git a/serial.hpp b/serial.hpp
index d708cbee..4e949a71 100644
--- a/serial.hpp
+++ b/serial.hpp
@@ -75,7 +75,7 @@ struct serial_feature {
 	bool dropped = false;
 };
 
-void serialize_feature(FILE *geomfile, serial_feature *sf, std::atomic<long long> *geompos, const char *fname, long long wx, long long wy);
+std::string serialize_feature(serial_feature *sf, long long wx, long long wy);
 serial_feature deserialize_feature(FILE *geoms, std::atomic<long long> *geompos_in, unsigned z, unsigned tx, unsigned ty, unsigned *initial_x, unsigned *initial_y);
 
 struct reader {
diff --git a/tile.cpp b/tile.cpp
index df6a2342..9d6372db 100644
--- a/tile.cpp
+++ b/tile.cpp
@@ -422,7 +422,9 @@ void rewrite(drawvec &geom, int z, int nextzoom, int maxzoom, long long *bbox, u
 						sf.values.push_back(metavals[i]);
 					}
 
-					serialize_feature(geomfile[j], &sf, &geompos[j], fname, SHIFT_RIGHT(initial_x[segment]), SHIFT_RIGHT(initial_y[segment]));
+					std::string feature = serialize_feature(&sf, SHIFT_RIGHT(initial_x[segment]), SHIFT_RIGHT(initial_y[segment]));
+					serialize_long_long(geomfile[j], feature.size(), &geompos[j], fname);
+					fwrite_check(feature.c_str(), sizeof(char), feature.size(), geomfile[j], &geompos[j], fname);
 				}
 			}
 		}