diff --git a/src/libspf2/spf_compile.c b/src/libspf2/spf_compile.c

index b08ffe2..ba3d804 100644

--- a/src/libspf2/spf_compile.c

+++ b/src/libspf2/spf_compile.c

@@ -604,7 +604,7 @@ SPF_c_parse_macro(SPF_server_t *spf_server,

 		default:

 			if (spf_server->debug > 3)

-				SPF_debugf("Adding illegal %%-follower '%c' at %d",

+				SPF_debugf("Adding illegal %%-follower '%c' at %zu",

 				src[idx], idx);

 			/* SPF spec says to treat it as a literal, not

 			 * SPF_E_INVALID_ESC */