diff --git a/Lib/imghdr.py b/Lib/imghdr.py
index f34f234..d0f66e5 100644
--- a/Lib/imghdr.py
+++ b/Lib/imghdr.py
@@ -36,14 +36,14 @@
 
 def test_rgb(h, f):
     """SGI image library"""
-    if h[:2] == '\001\332':
+    if h[:2] == b'\001\332':
         return 'rgb'
 
 tests.append(test_rgb)
 
 def test_gif(h, f):
     """GIF ('87 and '89 variants)"""
-    if h[:6] in ('GIF87a', 'GIF89a'):
+    if h[:6] in (b'GIF87a', b'GIF89a'):
         return 'gif'
 
 tests.append(test_gif)
@@ -51,7 +51,7 @@
 def test_pbm(h, f):
     """PBM (portable bitmap)"""
     if len(h) >= 3 and \
-        h[0] == 'P' and h[1] in '14' and h[2] in ' \t\n\r':
+        h[0] == ord('P') and h[1] in b'14' and h[2] in b' \t\n\r':
         return 'pbm'
 
 tests.append(test_pbm)
@@ -59,7 +59,7 @@
 def test_pgm(h, f):
     """PGM (portable graymap)"""
     if len(h) >= 3 and \
-        h[0] == 'P' and h[1] in '25' and h[2] in ' \t\n\r':
+        h[0] == ord('P') and h[1] in b'25' and h[2] in b' \t\n\r':
         return 'pgm'
 
 tests.append(test_pgm)
@@ -67,28 +67,28 @@
 def test_ppm(h, f):
     """PPM (portable pixmap)"""
     if len(h) >= 3 and \
-        h[0] == 'P' and h[1] in '36' and h[2] in ' \t\n\r':
+        h[0] == ord('P') and h[1] in b'36' and h[2] in b' \t\n\r':
         return 'ppm'
 
 tests.append(test_ppm)
 
 def test_tiff(h, f):
     """TIFF (can be in Motorola or Intel byte order)"""
-    if h[:2] in ('MM', 'II'):
+    if h[:2] in (b'MM', b'II'):
         return 'tiff'
 
 tests.append(test_tiff)
 
 def test_rast(h, f):
     """Sun raster file"""
-    if h[:4] == '\x59\xA6\x6A\x95':
+    if h[:4] == b'\x59\xA6\x6A\x95':
         return 'rast'
 
 tests.append(test_rast)
 
 def test_xbm(h, f):
     """X bitmap (X10 or X11)"""
-    s = '#define '
+    s = b'#define '
     if h[:len(s)] == s:
         return 'xbm'
 
@@ -96,26 +96,26 @@
 
 def test_jpeg(h, f):
     """JPEG data in JFIF format"""
-    if h[6:10] == 'JFIF':
+    if h[6:10] == b'JFIF':
         return 'jpeg'
 
 tests.append(test_jpeg)
 
 def test_exif(h, f):
     """JPEG data in Exif format"""
-    if h[6:10] == 'Exif':
+    if h[6:10] == b'Exif':
         return 'jpeg'
 
 tests.append(test_exif)
 
 def test_bmp(h, f):
-    if h[:2] == 'BM':
+    if h[:2] == b'BM':
         return 'bmp'
 
 tests.append(test_bmp)
 
 def test_png(h, f):
-    if h[:8] == "\211PNG\r\n\032\n":
+    if h[:8] == b'\211PNG\r\n\032\n':
         return 'png'
 
 tests.append(test_png)
@@ -159,3 +159,6 @@
                 print(what(filename))
             except IOError:
                 print('*** not found ***')
+
+if __name__ == '__main__':
+    test()