diff --git a/Sources/NIO/ByteBuffer-core.swift b/Sources/NIO/ByteBuffer-core.swift
index 730d086b1c5..0507cfc6c9e 100644
--- a/Sources/NIO/ByteBuffer-core.swift
+++ b/Sources/NIO/ByteBuffer-core.swift
@@ -383,11 +383,11 @@ public struct ByteBuffer {
         }
 
         var base = ensureCapacityAndReturnStorageBase(capacity: underestimatedByteCount)
-        var idx = 0
-        for b in bytes {
-            if idx >= underestimatedByteCount {
-                base = ensureCapacityAndReturnStorageBase(capacity: idx + 1)
-            }
+        var (iterator, idx) = UnsafeMutableBufferPointer(start: base, count: underestimatedByteCount).initialize(from: bytes)
+        assert(idx == underestimatedByteCount)
+        while let b = iterator.next() {
+            assert(S.self != String.UTF8View.self)
+            base = ensureCapacityAndReturnStorageBase(capacity: idx + 1)
             base[idx] = b
             idx += 1
         }
diff --git a/Tests/NIOTests/ByteBufferTest.swift b/Tests/NIOTests/ByteBufferTest.swift
index e448baf67b2..61e59cde539 100644
--- a/Tests/NIOTests/ByteBufferTest.swift
+++ b/Tests/NIOTests/ByteBufferTest.swift
@@ -1165,7 +1165,7 @@ class ByteBufferTest: XCTestCase {
 
     func testUnderestimatingSequenceWorks() throws {
         struct UnderestimatingSequence: Sequence {
-            let storage: [UInt8] = Array(0..<12)
+            let storage: [UInt8] = Array(0...255)
             typealias Element = UInt8
 
             public var indices: CountableRange<Int> {
@@ -1187,15 +1187,15 @@ class ByteBufferTest: XCTestCase {
         buf = self.allocator.buffer(capacity: 4)
         buf.clear()
         buf.write(bytes: UnderestimatingSequence())
-        XCTAssertEqual(12, buf.readableBytes)
-        for i in 0..<12 {
+        XCTAssertEqual(256, buf.readableBytes)
+        for i in 0..<256 {
             let actual = Int(buf.readInteger()! as UInt8)
             XCTAssertEqual(i, actual)
         }
         buf = self.allocator.buffer(capacity: 4)
         buf.set(bytes: UnderestimatingSequence(), at: 0)
         XCTAssertEqual(0, buf.readableBytes)
-        for i in 0..<12 {
+        for i in 0..<256 {
             let actual = Int(buf.getInteger(at: i)! as UInt8)
             XCTAssertEqual(i, actual)
         }