diff --git a/setup.py b/setup.py
index 39da8b8..3a58426 100644
--- a/setup.py
+++ b/setup.py
@@ -11,14 +11,18 @@ try:
 except Exception as e:
     print('ERROR: Failed to get version from "git describe --tags": %s: %s' % (type(e).__name__, e))
     from __init__ import __version__ as fallback_version
+
+    # Mark builds with unretrievable version (GitHub tarballs, etc.) as -dirty
+    if not fallback_version.endswith('-dirty'):
+        fallback_version += '-dirty'
+
     print('Using fallback version of %r.' % fallback_version)
     version = fallback_version
 
-else:
-    # Success. Write the version to disk.
-    with open('__init__.py', 'w') as f:
-        f.write('# Automatically generated by setup.py\n')
-        f.write('__version__ = %r\n' % version)
+# Write the version to disk.
+with open('__init__.py', 'w') as f:
+    f.write('# Automatically generated by setup.py\n')
+    f.write('__version__ = %r\n' % version)
 
 curdir = path.abspath(path.dirname(__file__))