diff --git a/pkg/sql/conn_executor_prepare.go b/pkg/sql/conn_executor_prepare.go
index 2da1e78528cd..f6bb0e0c5104 100644
--- a/pkg/sql/conn_executor_prepare.go
+++ b/pkg/sql/conn_executor_prepare.go
@@ -599,8 +599,10 @@ func (ex *connExecutor) execDescribe(
 			res.SetPortalOutput(ctx, portal.Stmt.Columns, portal.OutFormats)
 		}
 	default:
-		return retErr(errors.AssertionFailedf(
-			"unknown describe type: %s", errors.Safe(descCmd.Type)))
+		return retErr(pgerror.Newf(
+			pgcode.ProtocolViolation,
+			"invalid DESCRIBE message subtype %d", errors.Safe(byte(descCmd.Type)),
+		))
 	}
 	return nil, nil
 }
diff --git a/pkg/sql/pgwire/testdata/pgtest/portals b/pkg/sql/pgwire/testdata/pgtest/portals
index def589e3c10c..8a9a5123650f 100644
--- a/pkg/sql/pgwire/testdata/pgtest/portals
+++ b/pkg/sql/pgwire/testdata/pgtest/portals
@@ -1394,3 +1394,29 @@ ReadyForQuery
 {"Type":"DataRow","Values":[{"text":"2"}]}
 {"Type":"CommandComplete","CommandTag":"FETCH 2"}
 {"Type":"ReadyForQuery","TxStatus":"T"}
+
+send
+Query {"String": "ROLLBACK"}
+----
+
+until
+ReadyForQuery
+----
+{"Type":"CommandComplete","CommandTag":"ROLLBACK"}
+{"Type":"ReadyForQuery","TxStatus":"I"}
+
+send
+Parse {"Query": "SELECT * FROM generate_series(1, 4)"}
+Describe
+Bind
+Execute
+Sync
+----
+
+until keepErrMessage
+ErrorResponse
+ReadyForQuery
+----
+{"Type":"ParseComplete"}
+{"Type":"ErrorResponse","Code":"08P01","Message":"invalid DESCRIBE message subtype 0"}
+{"Type":"ReadyForQuery","TxStatus":"I"}