diff --git a/CMakeLists.txt b/CMakeLists.txt
index 236fd4d07c..2b068e0ce0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -48,6 +48,7 @@ endif()
 include_directories(.)
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 set(CMAKE_CXX_FLAGS "-std=c++11 -W -Wall -Wno-sign-compare")
+set(CMAKE_C_FLAGS "-std=c89 -W -Wall -Wno-sign-compare")
 
 if(APPLE)
   set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -undefined dynamic_lookup -flat_namespace")
diff --git a/tests/conformance_upb.c b/tests/conformance_upb.c
index a2e1d7a973..ccc8a94284 100644
--- a/tests/conformance_upb.c
+++ b/tests/conformance_upb.c
@@ -141,7 +141,7 @@ bool DoTestIo() {
   conformance_ConformanceResponse *response;
 
   if (!CheckedRead(STDIN_FILENO, &input_size, sizeof(uint32_t))) {
-    // EOF.
+    /* EOF. */
     return false;
   }
 
diff --git a/upb/bindings/lua/upb/pb.c b/upb/bindings/lua/upb/pb.c
index 15e8107354..bca2ee8546 100644
--- a/upb/bindings/lua/upb/pb.c
+++ b/upb/bindings/lua/upb/pb.c
@@ -29,7 +29,6 @@ static int lupb_pb_encode(lua_State *L) {
   const upb_msg *msg = lupb_msg_checkmsg2(L, 1, &layout);
   upb_arena arena;
   size_t size;
-  upb_status status = UPB_STATUS_INIT;
   char *result;
 
   upb_arena_init(&arena);