diff --git a/builder/main.py b/builder/main.py
index 6faed8e..d290f07 100644
--- a/builder/main.py
+++ b/builder/main.py
@@ -112,8 +112,9 @@ def fetch_spiffs_size(env):
             spiffs = p
     if not spiffs:
         sys.stderr.write(
-            env.subst("Could not find the `spiffs` section in the partitions "
-                      "table $PARTITIONS_TABLE_CSV\n"))
+            "Could not find the `spiffs` section in the partitions "
+            "table %s\n" % env.subst("$PARTITIONS_TABLE_CSV")
+        )
         env.Exit(1)
         return
     env["SPIFFS_START"] = _parse_size(spiffs['offset'])