diff --git a/internal/gumbo/tag.c b/internal/gumbo/tag.c
index 0663516c69..e7dce639e5 100644
--- a/internal/gumbo/tag.c
+++ b/internal/gumbo/tag.c
@@ -74,7 +74,7 @@ void gumbo_tag_from_original_text(GumboStringPiece* text) {
  * characters and is locale-independent to remain truthy to the
  * standard
  */
-#define tolower(c) gumbo_tolower(c)
+#define perfhash_tolower(c) gumbo_tolower(c)
 #include "tag_perf.h"
 
 static int
diff --git a/internal/gumbo/tag_perf.h b/internal/gumbo/tag_perf.h
index 7e4c6c380d..124d1ad092 100644
--- a/internal/gumbo/tag_perf.h
+++ b/internal/gumbo/tag_perf.h
@@ -89,7 +89,7 @@ static int T1[] = {
 };
 
 #ifndef perfhash_tolower
-#define perfhash_tolower(c) tolower(c)
+#define perfhash_tolower(c) gumbo_tolower(c)
 #endif
 
 static int perfhash(const unsigned char *key, int len)