diff --git a/config.m4 b/config.m4
index d4ec9a9..2f6d996 100644
--- a/config.m4
+++ b/config.m4
@@ -11,8 +11,8 @@
 
   LIBS="-lpthread $LIBS"
 
-  CFLAGS="-Wall -Werror -Wno-parentheses-equality -Wno-unused-value -std=c11 -g -O2"
-  CXXFLAGS="-std=c++11 -fno-exceptions -fno-rtti -g -O2"
+  CFLAGS="-Wall -Werror -Wno-parentheses-equality -Wno-unused-value -std=c11 -g -O2 -D PB_FIELD_16BIT=1"
+  CXXFLAGS="-std=c++11 -fno-exceptions -fno-rtti -g -O2 -D PB_FIELD_16BIT=1"
   GRPC_SHARED_LIBADD="-lpthread $GRPC_SHARED_LIBADD"
   PHP_REQUIRE_CXX()
   PHP_ADD_LIBRARY(pthread)