diff --git a/setup.py b/setup.py
index a5a62b9ab2..b203c316b3 100644
--- a/setup.py
+++ b/setup.py
@@ -14,18 +14,18 @@
 from setuptools import find_packages, setup
 import sys
 import os
-import copy
+
 
 # setting the 'CPPFLAGS' flag specifies the necessary cython dependency for "http-parser", for more info:
 # toil issue: https://github.com/DataBiosphere/toil/issues/2924
 # very similar to this issue: https://github.com/mcfletch/pyopengl/issues/11
 # the "right way" is waiting for a fix from "http-parser", but this fixes things in the meantime since that might take a while
-cppflags = os.environ.get('CPPFLAGS')
-if cppflags:
-    # note, duplicate options don't affect things here so we don't check - Mark D
-    os.environ['CPPFLAGS'] = ' '.join([cppflags, '-DPYPY_VERSION'])
-else:
-    os.environ['CPPFLAGS'] = '-DPYPY_VERSION'
+cppflags_args = [i.strip() for i in os.environ.get('CPPFLAGS', '').split(' ') if i.strip()]
+python_version = float('{}.{}'.format(str(sys.version_info.major), str(sys.version_info.minor)))
+if python_version >= 3.7 and '-DPYPY_VERSION' not in cppflags_args:
+    raise RuntimeError('Toil requires the environment variable "CPPFLAGS" to contain "-DPYPY_VERSION" when installed '
+                       'on python3.7 or higher.  This can be set with:\n\n'
+                       '    export CPPFLAGS=$CPPFLAGS" -DPYPY_VERSION"\n\n')
 
 
 def runSetup():
@@ -59,6 +59,11 @@ def runSetup():
     addict = 'addict<=2.2.0'
     sphinx = 'sphinx==1.7.5'
     pathlib2 = 'pathlib2==2.3.2'
+    # We need this specific http-parser that still claims to be version 0.8.3
+    # but which builds on Python 3.7+, to satisfy pymesos
+    http_parser = 'http-parser' + \
+                  '@https://github.com/adamnovak/http-parser/archive/190a17839ba229c635b59d960579451a81fe73f3.zip' + \
+                  '#sha256=3d30c84a426627e468657c44de199daee9d3210a48e392d4ad2e7497c5010949'
 
     core_reqs = [
         dill,
@@ -93,6 +98,7 @@ def runSetup():
     kubernetes_reqs = [
         kubernetes]
     mesos_reqs = [
+        http_parser,
         pymesos,
         psutil]
     wdl_reqs = []
diff --git a/version_template.py b/version_template.py
index 194cc8f0d0..68b71c4588 100644
--- a/version_template.py
+++ b/version_template.py
@@ -26,7 +26,7 @@
 #  - only import from the Python standard run-time library (you can't have any dependencies)
 
 
-baseVersion = '3.22.1a4'
+baseVersion = '3.22.1a5'
 cgcloudVersion = '1.6.0a1.dev393'
 dockerName = 'toil'