diff --git a/interp/src/flatten/flat_ir/cell_prototype.rs b/interp/src/flatten/flat_ir/cell_prototype.rs
index dce481e579..1097fe8c77 100644
--- a/interp/src/flatten/flat_ir/cell_prototype.rs
+++ b/interp/src/flatten/flat_ir/cell_prototype.rs
@@ -59,6 +59,7 @@ pub enum PrimType1 {
     UnsynSMult,
     UnsynSDiv,
     UnsynSMod,
+    Undef,
 }
 
 /// An enum for encoding FP primitives operator types
@@ -654,6 +655,14 @@ impl CellPrototype {
                     }
                 }
 
+                "undef" => {
+                    get_params![params; width: "WIDTH"];
+                    Self::SingleWidth {
+                        op: PrimType1::Undef,
+                        width: width.try_into().unwrap(),
+                    }
+                }
+
                 _ => CellPrototype::Unknown(
                     name.to_string(),
                     param_binding.clone(),
diff --git a/interp/src/flatten/primitives/builder.rs b/interp/src/flatten/primitives/builder.rs
index 2b93755c32..5c1b1c5a4b 100644
--- a/interp/src/flatten/primitives/builder.rs
+++ b/interp/src/flatten/primitives/builder.rs
@@ -96,6 +96,7 @@ pub fn build_primitive(
             PrimType1::UnsynSMod => {
                 Box::new(StdUnsynSmod::new(base_port, *width))
             }
+            PrimType1::Undef => Box::new(StdUndef::new(base_port, *width)),
         },
         CellPrototype::FixedPoint {
             op: _,
diff --git a/interp/src/flatten/primitives/combinational.rs b/interp/src/flatten/primitives/combinational.rs
index 5158fecc4f..5e824032e8 100644
--- a/interp/src/flatten/primitives/combinational.rs
+++ b/interp/src/flatten/primitives/combinational.rs
@@ -42,7 +42,7 @@ impl Primitive for StdConst {
     }
 
     fn has_comb(&self) -> bool {
-        false
+        true
     }
 
     fn has_stateful(&self) -> bool {
@@ -548,3 +548,18 @@ comb_primitive!(StdUnsynSmod[WIDTH](left [0], right [1]) -> (out [2]) {
             &left.as_signed(),
             &right.as_signed()), WIDTH)))
 });
+
+pub struct StdUndef(GlobalPortIdx);
+
+impl StdUndef {
+    pub fn new(base_port: GlobalPortIdx, _width: u32) -> Self {
+        Self(base_port)
+    }
+}
+
+impl Primitive for StdUndef {
+    fn exec_comb(&self, port_map: &mut PortMap) -> UpdateResult {
+        port_map.write_undef(self.0)?;
+        Ok(UpdateStatus::Unchanged)
+    }
+}