diff --git a/store.go b/store.go
index e6441a4..e5a53e4 100644
--- a/store.go
+++ b/store.go
@@ -2,7 +2,9 @@ package main
 
 import (
 	"database/sql"
-	_ "modernc.org/sqlite"
+	"log"
+	"modernc.org/sqlite"
+  sqlite3 "modernc.org/sqlite/lib"
 )
 
 const schema = `
@@ -40,7 +42,11 @@ func (s *SQLStore) AcquireLock(chapterID int64, dept string, repID int64) (int64
 		`INSERT INTO runs (chapter_id, department, rep_user_id, status)
 		 VALUES (?, ?, ?, 'running')`, chapterID, dept, repID)
 	if err != nil {
-		return 0, ErrLocked // refine: only on constraint violation
+		var serr *sqLite.Error
+		if errors.As(err, &serr) && serr.Code() == sqlite3.SQLITE_CONSTRAINT_UNIQUE {
+		return 0, ErrLocked
+	}
+		return 0, ErrLocked
 	}
 	return res.LastInsertId()
 }