diff --git a/workers/dockerfilebuild.py b/workers/dockerfilebuild.py
index 389e83cb8..564dd9f5f 100644
--- a/workers/dockerfilebuild.py
+++ b/workers/dockerfilebuild.py
@@ -45,7 +45,8 @@ def matches_system_error(status_str):
 
   for match in KNOWN_MATCHES:
     # 4 because we might have a Unix control code at the start.
-    if status_str.find(match[0:len(match) + 4]) <= 4: 
+    found = status_str.find(match[0:len(match) + 4])
+    if found >= 0 and found <= 4: 
       return True
 
   return False