diff --git a/lib/http_ntlm.c b/lib/http_ntlm.c
index bcb844208..c3ceadc35 100644
--- a/lib/http_ntlm.c
+++ b/lib/http_ntlm.c
@@ -83,6 +83,10 @@
 #      include <rand.h>
 #    endif
 
+#ifndef OPENSSL_VERSION_NUMBER
+#error "OPENSSL_VERSION_NUMBER not defined"
+#endif
+
 #if OPENSSL_VERSION_NUMBER < 0x00907001L
 #define DES_key_schedule des_key_schedule
 #define DES_cblock des_cblock
diff --git a/lib/ssluse.c b/lib/ssluse.c
index 38ad2b1ff..67fd99a38 100644
--- a/lib/ssluse.c
+++ b/lib/ssluse.c
@@ -76,6 +76,10 @@
 /* The last #include file should be: */
 #include "memdebug.h"
 
+#ifndef OPENSSL_VERSION_NUMBER
+#error "OPENSSL_VERSION_NUMBER not defined"
+#endif
+
 #if OPENSSL_VERSION_NUMBER >= 0x0090581fL
 #define HAVE_SSL_GET1_SESSION 1
 #else