diff --git a/plyfile.py b/plyfile.py
index c2a1e1d..6e4fa52 100644
--- a/plyfile.py
+++ b/plyfile.py
@@ -1298,7 +1298,11 @@ def _decode(s):
         """
         if isinstance(s, str):
             return s
-        return s.decode('ascii')
+        try:
+            return s.decode('ascii')
+        except UnicodeDecodeError:
+            raise PlyHeaderParseError(
+                "header contains non-ASCII characters")
 
     def __iter__(self):
         """
diff --git a/test/test_plyfile.py b/test/test_plyfile.py
index cf15d46..2930796 100644
--- a/test/test_plyfile.py
+++ b/test/test_plyfile.py
@@ -802,6 +802,13 @@ def test_header_parse_error(s, line):
     assert e.exc_val.line == line
 
 
+def test_header_non_ascii_bytes():
+    # a non-PLY binary file (e.g. a PNG) should raise PlyHeaderParseError,
+    # not a raw UnicodeDecodeError from the non-ASCII header bytes
+    with Raises(PlyHeaderParseError):
+        PlyData.read(BytesIO(b'\x89PNG\r\n\x1a\n' + b'\x00' * 40))
+
+
 invalid_arrays = [
     numpy.zeros((2, 2)),
     numpy.array([(0, (0, 0))],