diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4807db5..df17dce 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -190,7 +190,7 @@ if(SAFETYHOOK_AMALGAMATE) # amalgamate
 	add_custom_command(
 	    OUTPUT ${AMALGAMATED_FILE} ${AMALGAMATED_HEADER}
 	    DEPENDS ${HEADER_FILES} ${SOURCE_FILES} ${AMALGAMATE_SCRIPT}
-	    COMMAND ${Python3_EXECUTABLE} ${AMALGAMATE_SCRIPT} ${AMALGAMATED_FILE} ${AMALGAMATED_HEADER}
+	    COMMAND ${Python3_EXECUTABLE} ${AMALGAMATE_SCRIPT}
 	    MAIN_DEPENDENCY ${AMALGAMATE_SCRIPT}
 	    COMMENT "Amalgamating"
 	)
diff --git a/amalgamate.py b/amalgamate.py
index 2f2b51c..2a705cf 100644
--- a/amalgamate.py
+++ b/amalgamate.py
@@ -4,9 +4,12 @@
 from typing import List, Set
 from glob import glob
 from shutil import rmtree
+from textwrap import dedent
 
 import os
 import re
+import sys
+import argparse
 
 SAFETYHOOK_ROOT = Path(__file__).resolve().parent
 PUBLIC_INCLUDE_PATHS = [
@@ -18,6 +21,10 @@
 OUTPUT_DIR = SAFETYHOOK_ROOT / 'amalgamated-dist'
 FILE_HEADER = ['// DO NOT EDIT. This file is auto-generated by `amalgamate.py`.', '']
 
+parser = argparse.ArgumentParser(description='bundles cpp and hpp files together')
+parser.add_argument('--polyfill', action='store_true',
+                    help='replace std::except with a polyfill so it can be compiled on C++20 or older. https://raw.githubusercontent.com/TartanLlama/expected/master/include/tl/expected.hpp')
+
 
 # Python versions before 3.10 don't have the root_dir argument for glob, so we 
 # crudely emulate it here.
@@ -155,7 +162,25 @@ def merge_sources(*, source_dir: Path, covered_headers: Set[Path]):
     return output
 
 
+def do_polyfill(content):
+    return content.replace('#include <expected>',
+                           dedent('''
+                           #if __has_include("tl/expected.hpp")
+                               #include "tl/expected.hpp"
+                           #elif __has_include("expected.hpp")
+                               #include "expected.hpp"
+                           #else
+                               #error "No <expected> polyfill found"
+                           #endif
+                           ''')) \
+        .replace('std::expected', 'tl::expected') \
+        .replace('std::unexpected', 'tl::unexpected')
+
+
 def main():
+    args = parser.parse_args()
+    polyfill = args.polyfill is True
+
     if OUTPUT_DIR.exists():
         print('Output directory exists. Deleting.')
         rmtree(OUTPUT_DIR)
@@ -164,20 +189,26 @@ def main():
 
     covered_headers = set()
     with open(OUTPUT_DIR / 'safetyhook.hpp', 'w') as f:
-        f.write('\n'.join(FILE_HEADER + merge_headers(
+        content = '\n'.join(FILE_HEADER + merge_headers(
             header='safetyhook.hpp',
             search_paths=PUBLIC_INCLUDE_PATHS,
             covered_headers=covered_headers,
             stack=[],
-        )))
+        ))
+        if polyfill:
+            content = do_polyfill(content)
+        f.write(content)
 
     print(covered_headers)
 
     with open(OUTPUT_DIR / 'safetyhook.cpp', 'w') as f:
-        f.write('\n'.join(FILE_HEADER + merge_sources(
+        content = '\n'.join(FILE_HEADER + merge_sources(
             source_dir=SAFETYHOOK_ROOT / 'src',
             covered_headers=covered_headers,
-        )))
+        ))
+        if polyfill:
+            content = do_polyfill(content)
+        f.write(content)
 
 
 if __name__ == '__main__':
diff --git a/cmake.toml b/cmake.toml
index 70abb24..df17b9f 100644
--- a/cmake.toml
+++ b/cmake.toml
@@ -97,7 +97,7 @@ set(AMALGAMATE_SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/amalgamate.py)
 add_custom_command(
     OUTPUT ${AMALGAMATED_FILE} ${AMALGAMATED_HEADER}
     DEPENDS ${HEADER_FILES} ${SOURCE_FILES} ${AMALGAMATE_SCRIPT}
-    COMMAND ${Python3_EXECUTABLE} ${AMALGAMATE_SCRIPT} ${AMALGAMATED_FILE} ${AMALGAMATED_HEADER}
+    COMMAND ${Python3_EXECUTABLE} ${AMALGAMATE_SCRIPT}
     MAIN_DEPENDENCY ${AMALGAMATE_SCRIPT}
     COMMENT "Amalgamating"
 )