diff --git a/src/hyperloglog.c b/src/hyperloglog.c
index 8ccc16b..66cdaa8 100644
--- a/src/hyperloglog.c
+++ b/src/hyperloglog.c
@@ -598,6 +598,7 @@ int hllSparseToDense(robj *o) {
         } else {
             runlen = HLL_SPARSE_VAL_LEN(p);
             regval = HLL_SPARSE_VAL_VALUE(p);
+            if ((runlen + idx) > HLL_REGISTERS) break; /* Overflow. */
             while(runlen--) {
                 HLL_DENSE_SET_REGISTER(hdr->registers,idx,regval);
                 idx++;
@@ -1070,6 +1071,7 @@ int hllMerge(uint8_t *max, robj *hll) {
             } else {
                 runlen = HLL_SPARSE_VAL_LEN(p);
                 regval = HLL_SPARSE_VAL_VALUE(p);
+                if ((runlen + i) > HLL_REGISTERS) break; /* Overflow. */
                 while(runlen--) {
                     if (regval > max[i]) max[i] = regval;
                     i++;