diff --git a/m4/openssl.m4 b/m4/openssl.m4
index 0db50978..782e7e12 100644
--- a/m4/openssl.m4
+++ b/m4/openssl.m4
@@ -37,7 +37,7 @@ if test "$tinc_cv_openssl_lib" != "none given" ; then
   LIBS="$LIBS -L$tinc_cv_openssl_lib"
 fi
 if test "$tinc_cv_openssl_include" != "none given" ; then
-  INCLUDES="$INCLUDES -I$tinc_cv_openssl_include"
+  CFLAGS="$CFLAGS -I$tinc_cv_openssl_include"
 fi
 
 osi=found