diff --git a/lib/newfol/filemanip.py b/lib/newfol/filemanip.py
index 2c64cd6..e345c29 100644
--- a/lib/newfol/filemanip.py
+++ b/lib/newfol/filemanip.py
@@ -249,8 +249,9 @@ class HashHook(Hook):
                                                           line)
             except (IOError, FileNotFoundError):
                 if not self._options["forgiving"]:
-                    raise newfol.exception.UpgradeNeededError("dtb is missing"
-                                                              + " checksum")
+                    raise newfol.exception.UpgradeNeededError("dtb is " +
+                                                              "missing " +
+                                                              "checksum")
 
     def commit_close(self):
         if "w" in self._mode: