diff --git a/lib/curl_md5.h b/lib/curl_md5.h
index cd520bc9d081b38a5aa611209a36987e271bc4fc..ddc61e3e529880b151912414d57cf6bd0ae104a5 100644
--- a/lib/curl_md5.h
+++ b/lib/curl_md5.h
@@ -25,7 +25,7 @@
 #ifndef CURL_DISABLE_CRYPTO_AUTH
 #include "curl_hmac.h"
 
-const HMAC_params Curl_HMAC_MD5[1];
+extern const HMAC_params Curl_HMAC_MD5[1];
 
 void Curl_md5it(unsigned char *output,
                 const unsigned char *input);