diff --git a/configure.ac b/configure.ac
index 39a39d8f..5fa28987 100644
--- a/configure.ac
+++ b/configure.ac
@@ -432,7 +432,7 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
 #ifndef NATIVE_LITTLE_ENDIAN
 # error donna_c64 currently requires a little endian CPU
 #endif
-#ifdef EMSCRIPTEN
+#ifdef __EMSCRIPTEN__
 # error emscripten currently supports only shift operations on integers \
 #       larger than 64 bits
 #endif