diff --git a/apps/concat.C b/apps/concat.C
index 516834b914..7d50e544e8 100644
--- a/apps/concat.C
+++ b/apps/concat.C
@@ -77,7 +77,7 @@ void file_append(FILE* in, MFILE &out, int skip, int filenum) {
             boinc_checkpoint_completed();
         }
 
-        if (run_slow) sleep(1);
+        if (run_slow) boinc_sleep(1);
     }
 }