diff --git a/util/file_descriptor.hpp b/util/file_descriptor.hpp
index 51c125e..425a2bb 100644
--- a/util/file_descriptor.hpp
+++ b/util/file_descriptor.hpp
@@ -77,7 +77,7 @@
      * @return File descriptor.  Returns -1 if this object does not contain an
      *         open file descriptor.
      */
-    int operator()()
+    int operator()() const
     {
         return fd;
     }