diff --git a/tile.cpp b/tile.cpp
index d57003a4..e213407f 100644
--- a/tile.cpp
+++ b/tile.cpp
@@ -1367,6 +1367,10 @@ void *run_prefilter(void *v) {
 		layer_to_geojson(rpa->prefilter_fp, tmp_layer, 0, 0, 0, false, true);
 	}
 
+	if (fclose(rpa->prefilter_fp) != 0) {
+		perror("fclose output to prefilter");
+		exit(EXIT_FAILURE);
+	}
 	return NULL;
 }
 
@@ -1582,10 +1586,6 @@ long long write_tile(FILE *geoms, long long *geompos_in, char *metabase, char *s
 		}
 
 		if (prefilter != NULL) {
-			if (fclose(prefilter_fp) != 0) {
-				perror("fclose output to prefilter");
-				exit(EXIT_FAILURE);
-			}
 			if (close(prefilter_read) != 0) {
 				perror("close output from prefilter");
 				exit(EXIT_FAILURE);
diff --git a/write_json.cpp b/write_json.cpp
index f5c31735..e5b2287b 100644
--- a/write_json.cpp
+++ b/write_json.cpp
@@ -24,9 +24,9 @@ struct lonlat {
 	}
 };
 
-void layer_to_geojson(FILE *fp, mvt_layer &layer, unsigned z, unsigned x, unsigned y, bool comma, bool name) {
+void layer_to_geojson(FILE *fp, mvt_layer const &layer, unsigned z, unsigned x, unsigned y, bool comma, bool name) {
 	for (size_t f = 0; f < layer.features.size(); f++) {
-		mvt_feature &feat = layer.features[f];
+		mvt_feature const &feat = layer.features[f];
 
 		if (comma && f != 0) {
 			fprintf(fp, ",\n");
diff --git a/write_json.hpp b/write_json.hpp
index 685a8532..c7f1b2ad 100644
--- a/write_json.hpp
+++ b/write_json.hpp
@@ -1,2 +1,2 @@
-void layer_to_geojson(FILE *fp, mvt_layer &layer, unsigned z, unsigned x, unsigned y, bool comma, bool name);
+void layer_to_geojson(FILE *fp, mvt_layer const &layer, unsigned z, unsigned x, unsigned y, bool comma, bool name);
 void fprintq(FILE *f, const char *s);