diff --git a/src/frontend/src/expr/type_inference/cast.rs b/src/frontend/src/expr/type_inference/cast.rs
index cafa670a4ec5f..76ff7f307a573 100644
--- a/src/frontend/src/expr/type_inference/cast.rs
+++ b/src/frontend/src/expr/type_inference/cast.rs
@@ -27,7 +27,7 @@ use crate::expr::{Expr as _, ExprImpl};
 ///
 /// If you also need to cast them to this type, and there are more than 2 exprs, check out
 /// [`align_types`].
-pub fn least_restrictive(lhs: DataType, rhs: DataType) -> Result<DataType> {
+pub fn least_restrictive(lhs: DataType, rhs: DataType) -> std::result::Result<DataType, ErrorCode> {
     if lhs == rhs {
         Ok(lhs)
     } else if cast_ok(&lhs, &rhs, CastContext::Implicit) {
@@ -35,13 +35,18 @@ pub fn least_restrictive(lhs: DataType, rhs: DataType) -> Result<DataType> {
     } else if cast_ok(&rhs, &lhs, CastContext::Implicit) {
         Ok(lhs)
     } else {
-        Err(ErrorCode::BindError(format!("types {:?} and {:?} cannot be matched", lhs, rhs)).into())
+        Err(ErrorCode::BindError(format!(
+            "types {:?} and {:?} cannot be matched",
+            lhs, rhs
+        )))
     }
 }
 
 /// Find the `least_restrictive` type over a list of `exprs`, and add implicit cast when necessary.
 /// Used by `VALUES`, `CASE`, `UNION`, etc. See [PG](https://www.postgresql.org/docs/current/typeconv-union-case.html).
-pub fn align_types<'a>(exprs: impl Iterator<Item = &'a mut ExprImpl>) -> Result<DataType> {
+pub fn align_types<'a>(
+    exprs: impl Iterator<Item = &'a mut ExprImpl>,
+) -> std::result::Result<DataType, ErrorCode> {
     use std::mem::swap;
 
     let exprs = exprs.collect_vec();
@@ -60,7 +65,8 @@ pub fn align_types<'a>(exprs: impl Iterator<Item = &'a mut ExprImpl>) -> Result<
     for e in exprs {
         let mut dummy = ExprImpl::literal_bool(false);
         swap(&mut dummy, e);
-        *e = dummy.cast_implicit(ret_type.clone())?;
+        // unwrap: cast to least_restrictive type always succeeds
+        *e = dummy.cast_implicit(ret_type.clone()).unwrap();
     }
     Ok(ret_type)
 }
diff --git a/src/frontend/src/expr/type_inference/func.rs b/src/frontend/src/expr/type_inference/func.rs
index 65494e41afbf6..a9225ea0eaf49 100644
--- a/src/frontend/src/expr/type_inference/func.rs
+++ b/src/frontend/src/expr/type_inference/func.rs
@@ -118,6 +118,7 @@ fn infer_type_for_special(
                 }
             }))
             .map(Some)
+            .map_err(Into::into)
         }
         ExprType::In => {
             align_types(inputs.iter_mut())?;
@@ -125,7 +126,7 @@ fn infer_type_for_special(
         }
         ExprType::Coalesce => {
             ensure_arity!("coalesce", 1 <= | inputs |);
-            align_types(inputs.iter_mut()).map(Some)
+            align_types(inputs.iter_mut()).map(Some).map_err(Into::into)
         }
         ExprType::ConcatWs => {
             ensure_arity!("concat_ws", 2 <= | inputs |);