diff --git a/Makefile b/Makefile
index 32b3d573..9ba2c480 100644
--- a/Makefile
+++ b/Makefile
@@ -55,7 +55,7 @@ gh-pages: docs
 
 # Build amalgamation of all Wren library files.
 wren.c: src/include/wren.h src/vm/*.h src/vm/*.c
-	./script/generate_amalgamation.py $^ > $@
+	./script/generate_amalgamation.py > $@
 
 .DELETE_ON_ERROR: wren.c
 .PHONY: debug release all clean test builtin docs watchdocs gh-pages
diff --git a/script/generate_amalgamation.py b/script/generate_amalgamation.py
index 01faad6c..6f862e90 100755
--- a/script/generate_amalgamation.py
+++ b/script/generate_amalgamation.py
@@ -1,10 +1,12 @@
 #!/usr/bin/env python
 
 import sys
-from os.path import basename, dirname, join
+from os.path import basename, dirname, join, realpath
+from glob import iglob
 import re
 
 INCLUDE_PATTERN = re.compile(r'^\s*#include "([\w.]+)"')
+GUARD_PATTERN = re.compile(r'^#ifndef wren(_\w+)?_h$')
 WREN_DIR = dirname(dirname(realpath(__file__)))
 
 seen_files = set()
@@ -23,7 +25,7 @@ def add_file(filename):
   # Only include each file at most once.
   if bname in seen_files:
     return
-  seen_files.add(bname)
+  once = False
   path = dirname(filename)
 
   out.write('// Begin file "{0}"\n'.format(filename))
@@ -34,11 +36,21 @@ def add_file(filename):
         add_file(join(path, m.group(1)))
       else:
         out.write(line)
+      if GUARD_PATTERN.match(line):
+        once = True
   out.write('// End file "{0}"\n'.format(filename))
 
+  # Only skip header files which use #ifndef guards.
+  # This is necessary because of the X Macro technique.
+  if once:
+    seen_files.add(bname)
+
 # Print license on top.
 add_comment_file(join(WREN_DIR, 'LICENSE'))
 out.write('\n')
 # Source files.
-for f in sys.argv[1:]:
+add_file(join(WREN_DIR, 'src', 'include', 'wren.h'))
+# Must be included here because of conditional compilation.
+add_file(join(WREN_DIR, 'src', 'vm', 'wren_debug.h'))
+for f in iglob(join(WREN_DIR, 'src', 'vm', '*.c')):
   add_file(f)