diff --git a/glib/valgrind.h b/glib/valgrind.h
index ab76d41f3..a7f1f5655 100644
--- a/glib/valgrind.h
+++ b/glib/valgrind.h
@@ -134,8 +134,9 @@
       || defined(__CYGWIN32__) \
       || (defined(_WIN32) && defined(_M_IX86))
 #  define PLAT_x86_win32 1
-#elif (defined(__MINGW64__) && defined(__x86_64__)) \
-      || (defined(_WIN64) && defined(_M_X64))
+#elif (defined(__MINGW32__) && defined(__x86_64__)) \
+      || (defined(_WIN32) && defined(_M_X64))
+/* __MINGW32__ and _WIN32 are defined in 64 bit mode as well. */
 #  define PLAT_amd64_win64 1
 #elif defined(__linux__) && defined(__i386__)
 #  define PLAT_x86_linux 1