diff --git a/xinclude.c b/xinclude.c
index 9ba6c40..5181214 100644
--- a/xinclude.c
+++ b/xinclude.c
@@ -1064,7 +1064,8 @@
 	    if ((ent->SystemID != NULL) && (prev->SystemID != NULL)) {
 		if (!xmlStrEqual(ent->SystemID, prev->SystemID))
 		    goto error;
-	    } else if ((ent->SystemID != NULL) && (prev->SystemID != NULL)) {
+	    } else if ((ent->ExternalID != NULL) &&
+		       (prev->ExternalID != NULL)) {
 		if (!xmlStrEqual(ent->ExternalID, prev->ExternalID))
 		    goto error;
 	    } else if ((ent->content != NULL) && (prev->content != NULL)) {