diff --git a/qbase64.lisp b/qbase64.lisp
index bf0fbd3..f8446f5 100644
--- a/qbase64.lisp
+++ b/qbase64.lisp
@@ -58,10 +58,11 @@
                                (end1 (length bytes))
                                (start2 0)
                                (end2 (length string)))
-    (((bytes (simple-array (unsigned-byte 8))) (string simple-string))
+    (((bytes (simple-array (unsigned-byte 8))) (string simple-base-string))
+     ((bytes (simple-array (unsigned-byte 8))) (string simple-string))
      ((bytes (simple-array (unsigned-byte 8))) (string string))
-     ((bytes (array (unsigned-byte 8)))        (string string))
-     ((bytes (array (unsigned-byte 8)))        (string simple-string))
+     ((bytes array)                            (string simple-base-string))
+     ((bytes array)                            (string simple-string))
      ((bytes array)                            (string string)))
   (declare (type scheme scheme))
   (declare (type positive-fixnum start1 end1 start2 end2))