diff --git a/src/bio.c b/src/bio.c
index 4bd5a17c..27bc9abf 100644
--- a/src/bio.c
+++ b/src/bio.c
@@ -142,6 +142,13 @@ void *bioProcessBackgroundJobs(void *arg) {
     unsigned long type = (unsigned long) arg;
     sigset_t sigset;
 
+    /* Check that the type is within the right interval. */
+    if (type >= REDIS_BIO_NUM_OPS) {
+        redisLog(REDIS_WARNING,
+            "Warning: bio thread started with wrong type %lu",type);
+        return NULL;
+    }
+
     /* Make the thread killable at any time, so that bioKillThreads()
      * can work reliably. */
     pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);