diff --git a/sqlx-core/src/mssql/arguments.rs b/sqlx-core/src/mssql/arguments.rs
index 98e6de84be..23d9ecaa2d 100644
--- a/sqlx-core/src/mssql/arguments.rs
+++ b/sqlx-core/src/mssql/arguments.rs
@@ -65,6 +65,13 @@ impl MssqlArguments {
         T: Encode<'q, Mssql> + Type<Mssql>,
     {
         let ty = value.produces().unwrap_or_else(T::type_info);
+        // `DataType::Null` is required in TDS for null values, but the
+        // `sp_executesql` declaration needs the parameter's Rust type.
+        let declaration_ty = if ty.0.is_null() {
+            T::type_info()
+        } else {
+            ty.clone()
+        };
 
         // produce an ordinal parameter name
         //  @p1, @p2, ... @pN
@@ -91,7 +98,7 @@ impl MssqlArguments {
 
         declarations.push_str(name);
         declarations.push(' ');
-        ty.0.fmt(declarations);
+        declaration_ty.0.fmt(declarations);
 
         // write out the parameter
 
diff --git a/sqlx-core/src/mssql/types/bytes.rs b/sqlx-core/src/mssql/types/bytes.rs
index 48e0f71318..825d4f6cbe 100644
--- a/sqlx-core/src/mssql/types/bytes.rs
+++ b/sqlx-core/src/mssql/types/bytes.rs
@@ -8,7 +8,7 @@ use std::borrow::Cow;
 
 impl Type<Mssql> for [u8] {
     fn type_info() -> MssqlTypeInfo {
-        MssqlTypeInfo(TypeInfo::new(DataType::BigVarBinary, 0))
+        MssqlTypeInfo(TypeInfo::new(DataType::BigVarBinary, 0xFF_FF))
     }
 
     fn compatible(ty: &MssqlTypeInfo) -> bool {
diff --git a/sqlx-core/src/mssql/types/json.rs b/sqlx-core/src/mssql/types/json.rs
index 1dd67a2132..3f5aa3bfec 100644
--- a/sqlx-core/src/mssql/types/json.rs
+++ b/sqlx-core/src/mssql/types/json.rs
@@ -9,7 +9,7 @@ use serde::{Deserialize, Serialize};
 
 impl<T> Type<Mssql> for Json<T> {
     fn type_info() -> MssqlTypeInfo {
-        MssqlTypeInfo(TypeInfo::new(DataType::BigVarBinary, 0))
+        MssqlTypeInfo(TypeInfo::new(DataType::BigVarBinary, 0xFF_FF))
     }
 
     fn compatible(ty: &MssqlTypeInfo) -> bool {
diff --git a/tests/mssql/mssql.rs b/tests/mssql/mssql.rs
index 90cc8c81d4..cc4faf5701 100644
--- a/tests/mssql/mssql.rs
+++ b/tests/mssql/mssql.rs
@@ -193,6 +193,21 @@ async fn it_binds_empty_string_and_null() -> anyhow::Result<()> {
     Ok(())
 }
 
+// Regression test for https://github.com/sqlpage/sqlx-oldapi/issues/58
+#[sqlx_macros::test]
+async fn it_binds_null_with_type_in_isnull_expression() -> anyhow::Result<()> {
+    let mut conn = new::<Mssql>().await?;
+
+    let (val,): (String,) = sqlx_oldapi::query_as("SELECT ISNULL(@p1, N'hello')")
+        .bind(None::<String>)
+        .fetch_one(&mut conn)
+        .await?;
+
+    assert_eq!(val, "hello");
+
+    Ok(())
+}
+
 #[sqlx_macros::test]
 async fn it_binds_string_with_special_chars() -> anyhow::Result<()> {
     let mut conn = new::<Mssql>().await?;