diff --git a/src/fstapi.c b/src/fstapi.c
index 422c6ef..6251ff1 100644
--- a/src/fstapi.c
+++ b/src/fstapi.c
@@ -5086,10 +5086,11 @@ static int fstReaderParseChainTable(unsigned char *chain_cmem,
 
                 if (shval > 0) {
                     pval = chain_table[idx] = pval + shval;
-                    if (idx) {
+                    if (have_physical_chain) {
                         chain_table_lengths[pidx] = pval - chain_table[pidx];
                     }
                     pidx = idx++;
+		    have_physical_chain = 1;
                 } else if (shval < 0) {
                     chain_table[idx] =
                         0; /* need to explicitly zero as calloc above might not run */
@@ -5112,7 +5113,9 @@ static int fstReaderParseChainTable(unsigned char *chain_cmem,
                 }
 
                 for (i = 0; i < loopcnt; i++) {
-                    chain_table[idx++] = 0;
+                    chain_table[idx] = 0;
+                    chain_table_lengths[idx] = 0;
+                    idx++;
                 }
             }
 
@@ -5145,10 +5148,11 @@ static int fstReaderParseChainTable(unsigned char *chain_cmem,
                 }
 
                 pval = chain_table[idx] = pval + (val >> 1);
-                if (idx) {
+                if (have_physical_chain) {
                     chain_table_lengths[pidx] = pval - chain_table[pidx];
                 }
                 pidx = idx++;
+		have_physical_chain = 1;
             } else {
                 fstHandle loopcnt = val >> 1;
 
@@ -5157,7 +5161,9 @@ static int fstReaderParseChainTable(unsigned char *chain_cmem,
                 }
 
                 for (i = 0; i < loopcnt; i++) {
-                    chain_table[idx++] = 0;
+                    chain_table[idx] = 0;
+                    chain_table_lengths[idx] = 0;
+                    idx++;
                 }
             }
 
@@ -5170,7 +5176,8 @@ static int fstReaderParseChainTable(unsigned char *chain_cmem,
     /* the entry the room check kept back, so that the last chain a length was
        written to is as long as the distance from it to the end of the data */
     chain_table[idx] = vc_len;
-    chain_table_lengths[pidx] = chain_table[idx] - chain_table[pidx];
+    if (have_physical_chain)
+        chain_table_lengths[pidx] = chain_table[idx] - chain_table[pidx];
 
     for (i = 0; i < idx; i++) {
         int32_t v32 = chain_table_lengths[i];
@@ -5272,6 +5279,8 @@ int fstReaderIterBlocks2(fstReaderContext *ctx,
         uint32_t *tc_head = NULL;
         uint32_t tc_head_items = 0;
         traversal_mem_offs = 0;
+        int have_physical_chain = 0;
+        pidx = 0; /* current section only */
 
         fstReaderFseeko(xc, xc->f, blkpos, SEEK_SET);