diff --git a/optimization_engine/runner.py b/optimization_engine/runner.py
index 56056df1..f53a8ee0 100644
--- a/optimization_engine/runner.py
+++ b/optimization_engine/runner.py
@@ -178,7 +178,7 @@ class OptimizationRunner:
         # Update metadata
         if self.study:
             metadata['total_trials'] = len(self.study.trials)
-            metadata['last_updated': datetime.now().isoformat()
+            metadata['last_updated'] = datetime.now().isoformat()
             if not is_new and 'created_at' in metadata:
                 metadata['resume_count'] = metadata.get('resume_count', 0) + 1