diff --git a/sqlx-core/src/error.rs b/sqlx-core/src/error.rs
index 042342ef..17774add 100644
--- a/sqlx-core/src/error.rs
+++ b/sqlx-core/src/error.rs
@@ -312,11 +312,16 @@ impl From<crate::migrate::MigrateError> for Error {
 /// Format an error message as a `Protocol` error
 #[macro_export]
 macro_rules! err_protocol {
-    ($expr:expr) => {
-        $crate::error::Error::Protocol($expr.into())
-    };
-
-    ($fmt:expr, $($arg:tt)*) => {
-        $crate::error::Error::Protocol(format!($fmt, $($arg)*))
+    ($($fmt_args:tt)*) => {
+        $crate::error::Error::Protocol(
+            format!(
+                "{} ({}:{})",
+                // Note: the format string needs to be unmodified (e.g. by `concat!()`)
+                // for implicit formatting arguments to work
+                format_args!($($fmt_args)*),
+                module_path!(),
+                line!(),
+            )
+        )
     };
 }
diff --git a/sqlx-postgres/src/connection/describe.rs b/sqlx-postgres/src/connection/describe.rs
index d9c55201..9d532a51 100644
--- a/sqlx-postgres/src/connection/describe.rs
+++ b/sqlx-postgres/src/connection/describe.rs
@@ -466,7 +466,13 @@ WHERE rngtypid = $1
         let mut nullables: Vec<Option<bool>> = nullable_query
             .build_query_scalar()
             .fetch_all(&mut *self)
-            .await?;
+            .await
+            .map_err(|e| {
+                err_protocol!(
+                    "error from nullables query: {e}; query: {:?}",
+                    nullable_query.sql()
+                )
+            })?;
 
         // If the server is CockroachDB or Materialize, skip this step (#1248).
         if !self.stream.parameter_statuses.contains_key("crdb_version")
diff --git a/sqlx-postgres/src/io/buf_mut.rs b/sqlx-postgres/src/io/buf_mut.rs
index ff6fe03d..eea9d34a 100644
--- a/sqlx-postgres/src/io/buf_mut.rs
+++ b/sqlx-postgres/src/io/buf_mut.rs
@@ -27,7 +27,7 @@ impl PgBufMutExt for Vec<u8> {
         let size_result = write_result.and_then(|_| {
             let size = self.len() - offset;
             i32::try_from(size)
-                .map_err(|_| err_protocol!("message size out of range for Pg protocol: {size"))
+                .map_err(|_| err_protocol!("message size out of range for protocol: {size}"))
         });
 
         match size_result {