diff --git a/projection.cpp b/projection.cpp
index 7c973e88..194b25ee 100644
--- a/projection.cpp
+++ b/projection.cpp
@@ -150,7 +150,7 @@ void hilbert_d2xy(unsigned long long n, unsigned long long d, unsigned *x, unsig
 	}
 }
 
-unsigned long long encode_hilbert(unsigned int wx, unsigned int wy) {
+index_t encode_hilbert(unsigned int wx, unsigned int wy) {
 	return hilbert_xy2d(1LL << UINT_BITS, wx, wy);
 }
 
@@ -158,7 +158,7 @@ void decode_hilbert(index_t index, unsigned *wx, unsigned *wy) {
 	hilbert_d2xy(1LL << UINT_BITS, index, wx, wy);
 }
 
-unsigned long long encode_quadkey(unsigned int wx, unsigned int wy) {
+index_t encode_quadkey(unsigned int wx, unsigned int wy) {
 	unsigned long long out = 0;
 
 	int i;
diff --git a/projection.hpp b/projection.hpp
index 47983162..7aaed214 100644
--- a/projection.hpp
+++ b/projection.hpp
@@ -23,10 +23,10 @@ extern struct projection projections[];
 extern index_t (*encode_index)(unsigned int wx, unsigned int wy);
 extern void (*decode_index)(index_t index, unsigned *wx, unsigned *wy);
 
-unsigned long long encode_quadkey(unsigned int wx, unsigned int wy);
+index_t encode_quadkey(unsigned int wx, unsigned int wy);
 void decode_quadkey(index_t index, unsigned *wx, unsigned *wy);
 
-unsigned long long encode_hilbert(unsigned int wx, unsigned int wy);
+index_t encode_hilbert(unsigned int wx, unsigned int wy);
 void decode_hilbert(index_t index, unsigned *wx, unsigned *wy);
 
 unsigned coordinate_to_encodable(long long coord);
diff --git a/serial.cpp b/serial.cpp
index 4ed6ed75..b7dc5ef7 100644
--- a/serial.cpp
+++ b/serial.cpp
@@ -92,6 +92,11 @@ void serialize_ulong_long(std::string &out, unsigned long long zigzag) {
 	out.append(buf, s - buf);
 }
 
+void serialize_index(std::string &out, index_t zigzag) {
+	serialize_ulong_long(out, zigzag);
+	serialize_ulong_long(out, ((__uint128_t) zigzag) >> 64);
+}
+
 void serialize_long_long(std::string &out, long long n) {
 	unsigned long long zigzag = protozero::encode_zigzag64(n);
 
@@ -142,6 +147,16 @@ void deserialize_ulong_long(const char **f, unsigned long long *zigzag) {
 	}
 }
 
+void deserialize_index(const char **f, index_t *zigzag) {
+	unsigned long long bottom;
+	unsigned long long top;
+
+	deserialize_ulong_long(f, &bottom);
+	deserialize_ulong_long(f, &top);
+
+	*zigzag = (((__uint128_t) top) << 64) | bottom;
+}
+
 void deserialize_uint(const char **f, unsigned *n) {
 	unsigned long long v;
 	deserialize_ulong_long(f, &v);
@@ -209,10 +224,10 @@ std::string serialize_feature(serial_feature *sf, long long wx, long long wy) {
 	write_geometry(sf->geometry, s, wx, wy);
 
 	if (sf->index != 0) {
-		serialize_ulong_long(s, sf->index);
+		serialize_index(s, sf->index);
 	}
 	if (sf->label_point != 0) {
-		serialize_ulong_long(s, sf->label_point);
+		serialize_index(s, sf->label_point);
 	}
 	if (sf->extent != 0) {
 		serialize_long_long(s, sf->extent);
@@ -264,10 +279,10 @@ serial_feature deserialize_feature(std::string const &geoms, unsigned z, unsigne
 	sf.geometry = decode_geometry(&cp, z, tx, ty, sf.bbox, initial_x[sf.segment], initial_y[sf.segment]);
 
 	if (sf.layer & (1 << FLAG_INDEX)) {
-		deserialize_ulong_long(&cp, &sf.index);
+		deserialize_index(&cp, &sf.index);
 	}
 	if (sf.layer & (1 << FLAG_LABEL_POINT)) {
-		deserialize_ulong_long(&cp, &sf.label_point);
+		deserialize_index(&cp, &sf.label_point);
 	}
 	if (sf.layer & (1 << FLAG_EXTENT)) {
 		deserialize_long_long(&cp, &sf.extent);