diff --git a/Modules/md5.h b/Modules/md5.h
index 2b6d25b..13628df 100644
--- a/Modules/md5.h
+++ b/Modules/md5.h
@@ -37,6 +37,8 @@
 typedef unsigned short int UINT4;
 #elif INT_MAX == 2147483647
 typedef unsigned int UINT4;
+#else
+#error "Can't find a 4-byte integral type"
 #endif
 
 /* ========== End global.h; continue md5.h ========== */