diff --git a/posix/fd.cpp b/posix/fd.cpp
index d8e08865..091916f0 100644
--- a/posix/fd.cpp
+++ b/posix/fd.cpp
@@ -65,7 +65,7 @@ fd::fd (int _fd):
 
 ///////////////////////////////////////////////////////////////////////////////
 fd
-fd::dup (void)
+fd::dup (void) const
 {
     return dup (m_fd);
 }
diff --git a/posix/fd.hpp b/posix/fd.hpp
index d7cf888f..8e59b0e8 100644
--- a/posix/fd.hpp
+++ b/posix/fd.hpp
@@ -40,7 +40,7 @@ namespace cruft::posix {
         // possible; one should not be doing this unless it is absolutely
         // required.
         fd (const fd&) = delete;
-        fd dup (void);
+        fd dup (void) const;
         static fd dup (int);
 
         ~fd ();