diff --git a/ext/libxml/ruby_xml_html_parser_context.c b/ext/libxml/ruby_xml_html_parser_context.c
index 1a7afb0..97daec8 100644
--- a/ext/libxml/ruby_xml_html_parser_context.c
+++ b/ext/libxml/ruby_xml_html_parser_context.c
@@ -13,7 +13,7 @@
  */
 
 VALUE cXMLHtmlParserContext;
-ID IO_ATTR;
+extern ID IO_ATTR;
 
 static void rxml_html_parser_context_free(xmlParserCtxtPtr ctxt)
 {
diff --git a/ext/libxml/ruby_xml_parser_context.c b/ext/libxml/ruby_xml_parser_context.c
index 218459d..0096f9d 100644
--- a/ext/libxml/ruby_xml_parser_context.c
+++ b/ext/libxml/ruby_xml_parser_context.c
@@ -6,7 +6,7 @@
 #include "ruby_xml_parser_context.h"
 
 VALUE cXMLParserContext;
-ID IO_ATTR;
+extern ID IO_ATTR;
 
 /*
  * Document-class: LibXML::XML::Parser::Context