diff --git a/peak/assembler/assembler_util.py b/peak/assembler/assembler_util.py
index 779cb9f..707d467 100644
--- a/peak/assembler/assembler_util.py
+++ b/peak/assembler/assembler_util.py
@@ -118,6 +118,6 @@ def is_valid(opcode: AbstractBitVector) -> AbstractBit:
                 return ft.reduce(
                         operator.or_,
                         (f(opcode) for f in range_validators),
-                        opcode.get_family().Bit(1))
+                        opcode.get_family().Bit(0))
 
     return _validator_cache.setdefault(opcodes, is_valid)
diff --git a/peak/assembler2/assembler_util.py b/peak/assembler2/assembler_util.py
index 779cb9f..707d467 100644
--- a/peak/assembler2/assembler_util.py
+++ b/peak/assembler2/assembler_util.py
@@ -118,6 +118,6 @@ def is_valid(opcode: AbstractBitVector) -> AbstractBit:
                 return ft.reduce(
                         operator.or_,
                         (f(opcode) for f in range_validators),
-                        opcode.get_family().Bit(1))
+                        opcode.get_family().Bit(0))
 
     return _validator_cache.setdefault(opcodes, is_valid)