diff --git a/library/aes.c b/library/aes.c
index b55c08a..fa773ec 100644
--- a/library/aes.c
+++ b/library/aes.c
@@ -1866,7 +1866,7 @@
 #elif MBEDTLS_AESNI_HAVE_CODE == 2
         mbedtls_printf("  AES note: AESNI code present (intrinsics implementation).\n");
 #else
-#error Unrecognised value for MBEDTLS_AESNI_HAVE_CODE
+#error "Unrecognised value for MBEDTLS_AESNI_HAVE_CODE"
 #endif
         if (mbedtls_aesni_has_support(MBEDTLS_AESNI_AES)) {
             mbedtls_printf("  AES note: using AESNI.\n");