diff --git a/swh/loader/package/nixguix/loader.py b/swh/loader/package/nixguix/loader.py
--- a/swh/loader/package/nixguix/loader.py
+++ b/swh/loader/package/nixguix/loader.py
@@ -236,7 +236,7 @@
     version = int(sources["version"])
     if version != 1:
         raise ValueError(
-            "The sources structure version '%d' is not supported", sources["version"]
+            f"The sources structure version '{sources['version']}' is not supported"
         )
 
     # If a source doesn't contain required attributes, this source is