diff --git a/src/Npgsql/Internal/Converters/Primitive/NumericConverters.cs b/src/Npgsql/Internal/Converters/Primitive/NumericConverters.cs
index 714d3c49d4..79a82a1bfa 100644
--- a/src/Npgsql/Internal/Converters/Primitive/NumericConverters.cs
+++ b/src/Npgsql/Internal/Converters/Primitive/NumericConverters.cs
@@ -34,7 +34,7 @@ public override ValueTask<BigInteger> ReadAsync(PgReader reader, CancellationTok
     {
         // If we don't need a read and can read buffered we delegate to our sync read method which won't do IO in such a case.
         if (!reader.ShouldBuffer(reader.CurrentRemaining))
-            Read(reader);
+            return new(Read(reader));
 
         return AsyncCore(reader, cancellationToken);
 
diff --git a/test/Npgsql.Tests/BugTests.cs b/test/Npgsql.Tests/BugTests.cs
index 8d46522c0f..b3cd644afd 100644
--- a/test/Npgsql.Tests/BugTests.cs
+++ b/test/Npgsql.Tests/BugTests.cs
@@ -4,6 +4,7 @@
 using NUnit.Framework;
 using System;
 using System.Data;
+using System.Numerics;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
@@ -1391,4 +1392,30 @@ public async Task Bug4123()
         Assert.DoesNotThrowAsync(stream.FlushAsync);
         Assert.DoesNotThrow(stream.Flush);
     }
+
+    [Test, IssueLink("https://github.com/npgsql/npgsql/issues/6389")]
+    public async Task Composite_with_BigInteger([Values(CommandBehavior.Default, CommandBehavior.SequentialAccess)] CommandBehavior behavior)
+    {
+        await using var adminConnection = await OpenConnectionAsync();
+        var type = await GetTempTypeName(adminConnection);
+        await adminConnection.ExecuteNonQueryAsync($"CREATE TYPE {type} as (value numeric)");
+
+        var dataSourceBuilder = CreateDataSourceBuilder();
+        dataSourceBuilder.MapComposite<Composite_with_BigInteger_Composite>(type);
+        await using var dataSource = dataSourceBuilder.Build();
+        await using var connection = await dataSource.OpenConnectionAsync();
+
+        await using var cmd = connection.CreateCommand();
+        cmd.CommandText = $"SELECT ROW(1234567890::numeric)::{type} FROM generate_series(1, 8000)";
+        await using var reader = await cmd.ExecuteReaderAsync(behavior);
+        while (await reader.ReadAsync())
+        {
+            Assert.DoesNotThrowAsync(async () => await reader.GetFieldValueAsync<Composite_with_BigInteger_Composite>(0));
+        }
+    }
+
+    class Composite_with_BigInteger_Composite
+    {
+        public BigInteger Value { get; set; }
+    }
 }