diff --git a/fastparquet/dataframe.py b/fastparquet/dataframe.py
index 13db1be1..51ba40c4 100644
--- a/fastparquet/dataframe.py
+++ b/fastparquet/dataframe.py
@@ -107,7 +107,7 @@ def cat(col):
                 # funky pandas not-dtype
                 t = t.base
             if ("M" in str(t) or "time" in str(t)) and "[" not in str(t):
-                t = t + "[ns]"
+                t = str(t) + "[ns]"
             d = np.empty(0, dtype=t)
             if d.dtype.kind == "M" and str(col) in timezones:
                 try: