diff --git a/python/s2snow/compute_NOBS.py b/python/s2snow/compute_NOBS.py
index 8dd3f1cea2bdee44305bd6cc29a3dbeed23b7967..41bc04792e168ef5d809fc02375e40c7951ffeb3 100644
--- a/python/s2snow/compute_NOBS.py
+++ b/python/s2snow/compute_NOBS.py
@@ -41,7 +41,7 @@ def compute_NOBS(input_file, synthesis_name=None, output_dir=None):
     logging.debug("output_dir : %s", output_dir)
 
     if not os.path.isfile(input_file):
-        logging.error("Input file does not exist : {}", input_file)
+        logging.error("Input file does not exist : %s", input_file)
         return
 
     if output_dir is None: