diff --git a/src/cal_tools/calcat_interface2.py b/src/cal_tools/calcat_interface2.py
index bd985314157c537ccbdfa1b9a5dba93f0040c6a5..3fa0b616736d6d830ca580f02aafd79f6287a9f4 100644
--- a/src/cal_tools/calcat_interface2.py
+++ b/src/cal_tools/calcat_interface2.py
@@ -15,6 +15,10 @@ import requests
 from oauth2_xfel_client import Oauth2ClientBackend
 
 
+# Default address to connect to, only available internally
+CALCAT_PROXY_URL = "http://exflcalproxy.desy.de:8080/"
+
+
 class ModuleNameError(KeyError):
     def __init__(self, name):
         self.name = name
@@ -158,7 +162,7 @@ global_client = None
 def get_client():
     global global_client
     if global_client is None:
-        setup_client("http://exflcalproxy:8080/", None, None, None)
+        setup_client(CALCAT_PROXY_URL, None, None, None)
     return global_client
 
 
@@ -193,6 +197,20 @@ def setup_client(
         user_email=user_email,
     )
 
+    # Check we can connect to exflcalproxy
+    if oauth_client is None and base_url == CALCAT_PROXY_URL:
+        try:
+            # timeout=(connect_timeout, read_timeout)
+            global_client.get_request('me', timeout=(1, 5))
+        except requests.ConnectionError as e:
+            raise RuntimeError(
+                "Could not connect to calibration catalog proxy. This proxy allows "
+                "unauthenticated access inside the XFEL/DESY network. To look up "
+                "calibration constants from outside, you will need to create an Oauth "
+                "client ID & secret in the CalCat web interface. You will still not "
+                "be able to load constants without the constant store folder."
+            ) from e
+
 
 _default_caldb_root = ...