diff --git a/include/openssl/base.h b/include/openssl/base.h
index a8b441bf..e575069b 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -107,6 +107,10 @@ extern "C" {
 #elif defined(__myriad2__)
 #define OPENSSL_32_BIT
 #else
+/* Note BoringSSL only supports standard 32-bit and 64-bit two's-complement,
+ * little-endian architectures. Functions will not produce the correct answer
+ * on other systems. Run the crypto_test binary, notably
+ * crypto/compiler_test.cc, before adding a new architecture. */
 #error "Unknown target CPU"
 #endif