diff --git a/llama.cpp b/llama.cpp
index ce4d68f38..bf640bc02 100644
--- a/llama.cpp
+++ b/llama.cpp
@@ -1044,6 +1044,9 @@ struct llama_kv_cell {
 struct llama_kv_cache {
     bool has_shift = false;
 
+    // Note: The value of head isn't only used to optimize searching
+    // for a free KV slot. llama_decode_internal also uses it, so it
+    // cannot be freely changed after a slot has been allocated.
     uint32_t head = 0;
     uint32_t size = 0;
 
@@ -1301,6 +1304,8 @@ static bool llama_kv_cache_init(
 
 // find an empty slot of size "n_tokens" in the cache
 // updates the cache head
+// Note: On success, it's important that cache.head points
+// to the first cell of the slot.
 static bool llama_kv_cache_find_slot(
            struct llama_kv_cache & cache,
         const struct llama_batch & batch) {
@@ -4563,8 +4568,12 @@ static int llama_decode_internal(
 #endif
 
     // update the kv ring buffer
-    lctx.kv_self.head      += n_tokens;
     lctx.kv_self.has_shift  = false;
+    lctx.kv_self.head      += n_tokens;
+    // Ensure kv cache head points to a valid index.
+    if (lctx.kv_self.head >= lctx.kv_self.size) {
+        lctx.kv_self.head = 0;
+    }
 
 #ifdef GGML_PERF
     // print timing information per ggml operation (for debugging purposes)