Fix accidental multiindex

fastparquet/api.py

@@ -494,7 +494,7 @@ def __str__(self):
 
 
 def _pre_allocate(size, columns, categories, index, cs, dt, tz=None):
-    index = index or []
+    index = [index] if isinstance(index, str) else (index or [])
     cols = [c for c in columns if c not in index]
     categories = categories or {}
     cats = cs.copy()