diff --git a/src/PIL/PpmImagePlugin.py b/src/PIL/PpmImagePlugin.py
index 93ce3a4d2..e25b4bcec 100644
--- a/src/PIL/PpmImagePlugin.py
+++ b/src/PIL/PpmImagePlugin.py
@@ -49,15 +49,15 @@ class PpmImageFile(ImageFile.ImageFile):
     format = "PPM"
     format_description = "Pbmplus image"
 
-    def _read_magic(self, s=b""):
+    def _read_magic(self, magic=b""):
         while True:  # read until next whitespace
             c = self.fp.read(1)
             if c in B_WHITESPACE:
                 break
-            s = s + c
-            if len(s) > 6:  # exceeded max magic number length
+            magic += c
+            if len(magic) > 6:  # exceeded max magic number length
                 break
-        return s
+        return magic
 
     def _read_token(self, token=b""):
         def _ignore_comment():  # ignores rest of the line; stops at CR, LF or EOF