diff --git a/tools/c-index-test/CMakeLists.txt b/tools/c-index-test/CMakeLists.txt
index 7360656..113172a 100644
--- a/tools/c-index-test/CMakeLists.txt
+++ b/tools/c-index-test/CMakeLists.txt
@@ -5,7 +5,7 @@ add_clang_executable(c-index-test
 if(NOT MSVC)
   set_property(
     SOURCE c-index-test.c
-    PROPERTY COMPILE_FLAGS "-std=c89"
+    PROPERTY COMPILE_FLAGS "-std=gnu89"
     )
 endif()