diff --git a/resources/migrations/20170220210009-comment-png-storage.up.sql b/resources/migrations/20170220210009-comment-png-storage.up.sql
index 48a4dae..390c265 100644
--- a/resources/migrations/20170220210009-comment-png-storage.up.sql
+++ b/resources/migrations/20170220210009-comment-png-storage.up.sql
@@ -8,5 +8,5 @@ ALTER TABLE "issues" ADD UNIQUE ("issue_id");
 -- table for github PNG comment images
 CREATE TABLE issue_comment (
 id SERIAL PRIMARY KEY,
-issue_id INTEGER REFERENCES issues (issue_id),
+issue_id INTEGER REFERENCES issues (issue_id) UNIQUE,
 png_data bytea);
diff --git a/src/clj/commiteth/util/png_rendering.clj b/src/clj/commiteth/util/png_rendering.clj
index e26fef3..430ab0f 100644
--- a/src/clj/commiteth/util/png_rendering.clj
+++ b/src/clj/commiteth/util/png_rendering.clj
@@ -37,7 +37,7 @@
             :out-enc :bytes :in html)]
     (if (= 0 exit)
       out
-      (do (log/error "Failed to generate PNG file" err)
+      (do (log/error "Failed to generate PNG file" err exit out)
           nil))))