diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index 2fd9a9bb87879e04407e5c99ac4c7d74b2fcd78c..454c8b49d84b54cd4d5e105d39d3f65c820cac50 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -5,6 +5,7 @@
 #include "globals.hh"
 
 #include <cstdlib>
+#include <cstring>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
@@ -160,7 +161,7 @@ void LocalStore::optimisePath_(OptimiseStats & stats, const Path & path, InodeHa
                full.  When that happens, it's fine to ignore it: we
                just effectively disable deduplication of this
                file.  */
-            printInfo("cannot link ‘%s’ to ‘%s’: %m", linkPath, path);
+            printInfo("cannot link ‘%s’ to ‘%s’: %s", linkPath, path, strerror(errno));
             return;
 
         default: