diff --git a/whois/settings.py b/whois/settings.py
index 22f13e9..3c49619 100644
--- a/whois/settings.py
+++ b/whois/settings.py
@@ -43,4 +43,6 @@
 
 # production
 # ip_mask = "192.168.88.1-255"
-ip_mask = os.environ.get("APP_IP_MASK", "192.168.88.1-255")
\ No newline at end of file
+ip_mask = os.environ.get("APP_IP_MASK", None)
+if not ip_mask:
+    raise ValueError("ERROR: APP_IP_MASK environment variable was not set!")
\ No newline at end of file