diff --git a/PIL/PngImagePlugin.py b/PIL/PngImagePlugin.py
index d556360a9..ace71d21d 100644
--- a/PIL/PngImagePlugin.py
+++ b/PIL/PngImagePlugin.py
@@ -550,11 +550,11 @@ def _save(im, fp, filename, chunk=putchunk, check=0):
 
     if "transparency" in im.encoderinfo:
         if im.mode == "P":
-            transparency = max(0, min(255, im.encoderinfo["transparency"]))
-            alpha = b'\xFF' * transparency + b'\0'
-            # limit to actual palette size
-            alpha_bytes = 2**bits
-            chunk(fp, b"tRNS", alpha[:alpha_bytes])
+            if isinstance(im.encoderinfo["transparency"], bytes):
+                chunk(fp, b"tRNS", b'\xFF' + im.encoderinfo["transparency"] + b'\0')
+            else:
+                transparency = max(0, min(255, im.encoderinfo["transparency"]))
+                chunk(fp, b"tRNS", b'\xFF' + transparency + b'\0')
         elif im.mode == "L":
             transparency = max(0, min(65535, im.encoderinfo["transparency"]))
             chunk(fp, b"tRNS", o16(transparency))