diff --git a/include/jemalloc/internal/sz.h b/include/jemalloc/internal/sz.h
index 69625ee2..68e558ab 100644
--- a/include/jemalloc/internal/sz.h
+++ b/include/jemalloc/internal/sz.h
@@ -122,6 +122,10 @@ sz_size2index_compute(size_t size) {
 	if (unlikely(size > SC_LARGE_MAXCLASS)) {
 		return SC_NSIZES;
 	}
+
+	if (size == 0) {
+		return 0;
+	}
 #if (SC_NTINY != 0)
 	if (size <= (ZU(1) << SC_LG_TINY_MAXCLASS)) {
 		szind_t lg_tmin = SC_LG_TINY_MAXCLASS - SC_NTINY + 1;
@@ -150,14 +154,14 @@ sz_size2index_compute(size_t size) {
 JEMALLOC_ALWAYS_INLINE szind_t
 sz_size2index_lookup(size_t size) {
 	assert(size <= SC_LOOKUP_MAXCLASS);
-	szind_t ret = (sz_size2index_tab[(size-1) >> SC_LG_TINY_MIN]);
+	szind_t ret = (sz_size2index_tab[(size + (ZU(1) << SC_LG_TINY_MIN) - 1)
+					 >> SC_LG_TINY_MIN]);
 	assert(ret == sz_size2index_compute(size));
 	return ret;
 }
 
 JEMALLOC_ALWAYS_INLINE szind_t
 sz_size2index(size_t size) {
-	assert(size > 0);
 	if (likely(size <= SC_LOOKUP_MAXCLASS)) {
 		return sz_size2index_lookup(size);
 	}
@@ -208,6 +212,10 @@ sz_s2u_compute(size_t size) {
 	if (unlikely(size > SC_LARGE_MAXCLASS)) {
 		return 0;
 	}
+
+	if (size == 0) {
+		size++;
+	}
 #if (SC_NTINY > 0)
 	if (size <= (ZU(1) << SC_LG_TINY_MAXCLASS)) {
 		size_t lg_tmin = SC_LG_TINY_MAXCLASS - SC_NTINY + 1;
@@ -241,7 +249,6 @@ sz_s2u_lookup(size_t size) {
  */
 JEMALLOC_ALWAYS_INLINE size_t
 sz_s2u(size_t size) {
-	assert(size > 0);
 	if (likely(size <= SC_LOOKUP_MAXCLASS)) {
 		return sz_s2u_lookup(size);
 	}
diff --git a/src/sz.c b/src/sz.c
index 77f89c62..8633fb05 100644
--- a/src/sz.c
+++ b/src/sz.c
@@ -37,18 +37,19 @@ sz_boot_index2size_tab(const sc_data_t *sc_data) {
  * the smallest interval for which the result can change.
  */
 JEMALLOC_ALIGNED(CACHELINE)
-uint8_t sz_size2index_tab[SC_LOOKUP_MAXCLASS >> SC_LG_TINY_MIN];
+uint8_t sz_size2index_tab[(SC_LOOKUP_MAXCLASS >> SC_LG_TINY_MIN) + 1];
 
 static void
 sz_boot_size2index_tab(const sc_data_t *sc_data) {
-	size_t dst_max = (SC_LOOKUP_MAXCLASS >> SC_LG_TINY_MIN);
+	size_t dst_max = (SC_LOOKUP_MAXCLASS >> SC_LG_TINY_MIN) + 1;
 	size_t dst_ind = 0;
 	for (unsigned sc_ind = 0; sc_ind < SC_NSIZES && dst_ind < dst_max;
 	    sc_ind++) {
 		const sc_t *sc = &sc_data->sc[sc_ind];
 		size_t sz = (ZU(1) << sc->lg_base)
 		    + (ZU(sc->ndelta) << sc->lg_delta);
-		size_t max_ind = ((sz - 1) >> SC_LG_TINY_MIN);
+		size_t max_ind = ((sz + (ZU(1) << SC_LG_TINY_MIN) - 1)
+				   >> SC_LG_TINY_MIN);
 		for (; dst_ind <= max_ind && dst_ind < dst_max; dst_ind++) {
 			sz_size2index_tab[dst_ind] = sc_ind;
 		}