diff --git a/bagit.py b/bagit.py
index 458fba8..9fff6e5 100755
--- a/bagit.py
+++ b/bagit.py
@@ -785,10 +785,9 @@ def _validate_contents(self, processes=1, fast=False, completeness_only=False):
                 _("Fast validation requires bag-info.txt to include Payload-Oxum")
             )
 
-        # Perform the fast file count + size check so we can fail early:
-        self._validate_oxum()
-
         if fast:
+            # Perform the fast file count + size check so we can fail early:
+            self._validate_oxum()
             return
 
         self._validate_completeness()