diff --git a/crates/circuits/primitives/src/encoder/mod.rs b/crates/circuits/primitives/src/encoder/mod.rs
index 2c44b27eda..b15b0e9384 100644
--- a/crates/circuits/primitives/src/encoder/mod.rs
+++ b/crates/circuits/primitives/src/encoder/mod.rs
@@ -13,6 +13,7 @@ pub struct Encoder {
     /// The maximal degree of the equalities in the AIR, however, **is one higher:** that is, `max_flag_degree + 1`.
     max_flag_degree: u32,
     pts: Vec<Vec<u32>>,
+    reserve_invalid: bool,
 }
 
 impl Encoder {
@@ -21,7 +22,7 @@ impl Encoder {
     /// The zero point is reserved for the dummy row.
     /// `max_degree` is the upper bound for the flag expressions, but the `eval` function
     /// of the encoder itself will use some constraints of degree `max_degree + 1`.
-    pub fn new(cnt: usize, max_degree: u32) -> Self {
+    pub fn new(cnt: usize, max_degree: u32, reserve_invalid: bool) -> Self {
         let binomial = |x: u32| {
             let mut res = 1;
             for i in 1..=max_degree {
@@ -52,6 +53,7 @@ impl Encoder {
             flag_cnt: cnt,
             max_flag_degree: max_degree,
             pts,
+            reserve_invalid,
         }
     }
 
@@ -86,13 +88,19 @@ impl Encoder {
         flag_idx: usize,
         vars: &[AB::Var],
     ) -> AB::Expr {
-        assert!(flag_idx <= self.flag_cnt, "flag index out of range");
-        self.expression_for_point::<AB>(&self.pts[flag_idx], vars)
+        assert!(
+            flag_idx + self.reserve_invalid as usize <= self.flag_cnt,
+            "flag index out of range"
+        );
+        self.expression_for_point::<AB>(&self.pts[flag_idx + self.reserve_invalid as usize], vars)
     }
 
     pub fn get_flag_pt(&self, flag_idx: usize) -> Vec<u32> {
-        assert!(flag_idx <= self.flag_cnt, "flag index out of range");
-        self.pts[flag_idx].clone()
+        assert!(
+            flag_idx + self.reserve_invalid as usize <= self.flag_cnt,
+            "flag index out of range"
+        );
+        self.pts[flag_idx + self.reserve_invalid as usize].clone()
     }
 
     pub fn is_valid<AB: InteractionBuilder>(&self, vars: &[AB::Var]) -> AB::Expr {
diff --git a/crates/circuits/sha256-air/src/sha256/air.rs b/crates/circuits/sha256-air/src/sha256/air.rs
index 451f8ad03e..fd0590a907 100644
--- a/crates/circuits/sha256-air/src/sha256/air.rs
+++ b/crates/circuits/sha256-air/src/sha256/air.rs
@@ -31,7 +31,7 @@ impl Sha256Air {
     pub fn new(bitwise_lookup_bus: BitwiseOperationLookupBus, self_bus_idx: usize) -> Self {
         Self {
             bitwise_lookup_bus,
-            row_idx_encoder: Encoder::new(17, 2),
+            row_idx_encoder: Encoder::new(17, 2, false),
             bus_idx: self_bus_idx,
         }
     }
diff --git a/crates/vm/src/system/public_values/core.rs b/crates/vm/src/system/public_values/core.rs
index aa2e7866bb..3b6b98a821 100644
--- a/crates/vm/src/system/public_values/core.rs
+++ b/crates/vm/src/system/public_values/core.rs
@@ -34,7 +34,7 @@ impl PublicValuesCoreAir {
         Self {
             num_custom_pvs,
             offset,
-            encoder: Encoder::new(num_custom_pvs, max_degree),
+            encoder: Encoder::new(num_custom_pvs, max_degree, true),
         }
     }
 }
diff --git a/extensions/sha256/circuit/src/sha256_chip/mod.rs b/extensions/sha256/circuit/src/sha256_chip/mod.rs
index 1256099cf3..200abd02b4 100644
--- a/extensions/sha256/circuit/src/sha256_chip/mod.rs
+++ b/extensions/sha256/circuit/src/sha256_chip/mod.rs
@@ -87,7 +87,7 @@ impl<F: PrimeField32> Sha256VmChip<F> {
                 ptr_max_bits,
                 offset,
                 Sha256Air::new(bitwise_lookup_chip.bus(), SHA256_CHIP_BUS_IDX),
-                Encoder::new(PaddingFlags::COUNT, 2),
+                Encoder::new(PaddingFlags::COUNT, 2, false),
             ),
             memory_controller,
             bitwise_lookup_chip,