diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 1de7f05..8cf0371 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -194,7 +194,7 @@
         /*
          * Increase counter
          */
-        for( i = CTR_DRBG_BLOCKSIZE; i >= 0; i-- )
+        for( i = CTR_DRBG_BLOCKSIZE; i > 0; i-- )
             if( ++ctx->counter[i - 1] != 0 )
                 break;