diff --git a/dos/str/strrev.k b/dos/str/strrev.k
index 246d212..be2ff88 100644
--- a/dos/str/strrev.k
+++ b/dos/str/strrev.k
@@ -9,6 +9,9 @@ strrev:
     cmovz b[ax0], 0
     jz .4
 
+    ; save str's location
+    mov rdx, ax1
+
 .1:
     test b[ax1+1], b[ax1+1]
     jz .2
@@ -19,8 +22,8 @@ strrev:
 .2:
     mov b[ax0], b[ax1]
 
-    test b[ax1], b[ax1]
-    jz .3
+    cmp ax1, rdx
+    je .3
 
     inc ax0
     dec ax1