diff --git a/setup.py b/setup.py
index 1b5f513..0e6208f 100644
--- a/setup.py
+++ b/setup.py
@@ -184,7 +184,7 @@ class type_generator(build_ext):
                 f"unable to find tss2_tpm2_types.h in {pk['include_dirs']}"
             )
         pdata = preprocess_file(
-            header_path, cpp_args=["-D__extension__=", "-D__attribute__(x)="]
+            header_path, cpp_args=["-std=c99", "-D__extension__=", "-D__attribute__(x)="]
         )
         parser = c_parser.CParser()
         ast = parser.parse(pdata, "tss2_tpm2_types.h")
@@ -205,6 +205,7 @@ class type_generator(build_ext):
                 pdata = preprocess_file(
                     policy_header_path,
                     cpp_args=[
+                        "-std=c99",
                         "-D__extension__=",
                         "-D__attribute__(x)=",
                         "-D__float128=long double",