diff --git a/std/utf.d b/std/utf.d
index e6081403fdf..62faf8e013a 100644
--- a/std/utf.d
+++ b/std/utf.d
@@ -2719,7 +2719,7 @@ auto byCodeUnit(R)(R r) if (isNarrowString!R)
         void popFront()                      { r = r[1 .. $]; }
         auto ref opIndex(size_t index) inout { return r[index]; }
 
-        @property auto ref back() const
+        @property auto ref back() inout
         {
             return r[$ - 1];
         }
@@ -2749,6 +2749,8 @@ auto byCodeUnit(R)(R r) if (isNarrowString!R)
         R r;
     }
 
+    static assert(isRandomAccessRange!ByCodeUnitImpl);
+
     return ByCodeUnitImpl(r);
 }