diff --git a/src/hyperloglog.c b/src/hyperloglog.c
index c0f20815c..6b18d1417 100644
--- a/src/hyperloglog.c
+++ b/src/hyperloglog.c
@@ -698,6 +698,7 @@ int hllSparseSet(robj *o, long index, uint8_t count) {
         first += span;
     }
     if (span == 0) return -1; /* Invalid format. */
+    if (span >= end) return -1; /* Invalid format. */
 
     next = HLL_SPARSE_IS_XZERO(p) ? p+2 : p+1;
     if (next >= end) next = NULL;