diff --git a/src/pylorax/executils.py b/src/pylorax/executils.py
index 4d7d6110..31b644ce 100644
--- a/src/pylorax/executils.py
+++ b/src/pylorax/executils.py
@@ -192,7 +192,7 @@ def _run_program(argv, root='/', stdin=None, stdout=None, env_prune=None, log_ou
         raise
 
     with program_log_lock:
-        program_log.debug("Return code: %d", proc.returncode)
+        program_log.debug("Return code: %s", proc.returncode)
 
     if proc.returncode and raise_err:
         output = output_string or "" + err_string or ""