diff --git a/wcs/qommon/substitution.py b/wcs/qommon/substitution.py
index d06ca95d0..b2bee529e 100644
--- a/wcs/qommon/substitution.py
+++ b/wcs/qommon/substitution.py
@@ -163,7 +163,7 @@ class CompatibilityNamesDict(dict):
     # custom dictionary that provides automatic fallback to legacy variable
     # names (namespaced with underscores)
 
-    valid_key_regex = re.compile(r'^[a-zA-Z][a-zA-Z0-9_]*$')
+    valid_key_regex = re.compile(r'^[a-zA-Z_][a-zA-Z0-9_]*$')
 
     def get(self, key, default=None):
         try: