diff --git a/tools/backend_lib.cpp b/tools/backend_lib.cpp
index 18b4410627..a7d8acab2d 100644
--- a/tools/backend_lib.cpp
+++ b/tools/backend_lib.cpp
@@ -164,7 +164,7 @@ int create_result(
 
     result.priority += priority_increase;
     sprintf(result.name, "%s_%s", wu.name, result_name_suffix);
-    sprintf(base_outfile_name, "%s_r%ld", result.name, lrand48());
+    sprintf(base_outfile_name, "%s_r%ld_", result.name, lrand48());
     retval = read_filename(
         result_template_filename, result_template, sizeof(result_template)
     );