diff --git a/src/gen-def.py b/src/gen-def.py
index 9a997d65a..ba39eaae5 100755
--- a/src/gen-def.py
+++ b/src/gen-def.py
@@ -4,8 +4,14 @@ from __future__ import print_function, division, absolute_import
 
 import io, os, re, sys
 
+if len (sys.argv) < 3:
+	sys.exit("usage: gen-def.py harfbuzz.def hb.h [hb-blob.h hb-buffer.h ...]")
+
+output_file = sys.argv[1]
+header_paths = sys.argv[2:]
+
 headers_content = []
-for h in os.environ["headers"].split (' '):
+for h in header_paths:
 	if h.endswith (".h"):
 		with io.open (h, encoding='utf-8') as f: headers_content.append (f.read ())
 
@@ -13,7 +19,7 @@ result = """EXPORTS
 %s
 LIBRARY lib%s-0.dll""" % (
 	"\n".join (sorted (re.findall (r"^hb_\w+(?= \()", "\n".join (headers_content), re.M))),
-	sys.argv[1].replace ('.def', '')
+	output_file.replace ('.def', '')
 )
 
-with open (sys.argv[1], "w") as f: f.write (result)
+with open (output_file, "w") as f: f.write (result)