diff --git a/setup.py b/setup.py
index 3d250e7249d..a6fcc12b84d 100644
--- a/setup.py
+++ b/setup.py
@@ -1765,7 +1765,9 @@ class PyBuildExt(build_ext):
                 ('XML_POOR_ENTROPY', '1'),
             ]
             extra_compile_args = []
-            expat_lib = []
+            # bpo-44394: libexpat uses isnan() of math.h and needs linkage
+            # against the libm
+            expat_lib = ['m']
             expat_sources = ['expat/xmlparse.c',
                              'expat/xmlrole.c',
                              'expat/xmltok.c']