diff -r 14c9284a8bad roundup/backends/rdbms_common.py
--- a/roundup/backends/rdbms_common.py	Wed Mar 17 10:54:14 2021 +0100
+++ b/roundup/backends/rdbms_common.py	Thu Mar 18 16:35:42 2021 +0100
@@ -2447,6 +2447,8 @@
             match the given expression.
             Returns tuple of 'WHERE' introns for the overall filter.
         """
+        if len(v) == 0:
+            return "TRUE", v
         try:
             opcodes = [int(x) for x in v]
             if min(opcodes) >= -1: raise ValueError()