diff --git a/gitdiff/apply.go b/gitdiff/apply.go
index 4397d99..05a4526 100644
--- a/gitdiff/apply.go
+++ b/gitdiff/apply.go
@@ -231,10 +231,7 @@ func (a *Applier) ApplyTextFragment(dst io.Writer, f *TextFragment) error {
 
 	preimage := make([][]byte, fragEnd-start)
 	n, err := a.lineSrc.ReadLinesAt(preimage, start)
-	switch {
-	case err == nil:
-	case err == io.EOF && n == len(preimage): // last line of frag has no newline character
-	default:
+	if err != nil {
 		return applyError(err, lineNum(start+int64(n)))
 	}
 
diff --git a/gitdiff/io.go b/gitdiff/io.go
index af5c847..8143238 100644
--- a/gitdiff/io.go
+++ b/gitdiff/io.go
@@ -5,21 +5,23 @@ import (
 	"io"
 )
 
+const (
+	byteBufferSize  = 32 * 1024 // from io.Copy
+	lineBufferSize  = 32
+	indexBufferSize = 1024
+)
+
 // LineReaderAt is the interface that wraps the ReadLinesAt method.
 //
-// ReadLinesAt reads len(lines) into lines starting at line offset in the
-// input source. It returns number of full lines read (0 <= n <= len(lines))
-// and any error encountered. Line numbers are zero-indexed.
+// ReadLinesAt reads len(lines) into lines starting at line offset. It returns
+// the number of lines read (0 <= n <= len(lines)) and any error encountered.
+// Line numbers are zero-indexed.
 //
 // If n < len(lines), ReadLinesAt returns a non-nil error explaining why more
 // lines were not returned.
 //
-// Each full line includes the line ending character(s). If the last line of
-// the input does not have a line ending character, ReadLinesAt returns the
-// content of the line and io.EOF.
-//
-// If the content of the input source changes after the first call to
-// ReadLinesAt, the behavior of future calls is undefined.
+// Lines read by ReadLinesAt include the newline character. The last line does
+// not have a final newline character if the input ends without one.
 type LineReaderAt interface {
 	ReadLinesAt(lines [][]byte, offset int64) (n int, err error)
 }
@@ -65,7 +67,7 @@ func (r *lineReaderAt) ReadLinesAt(lines [][]byte, offset int64) (n int, err err
 		lines[n] = buf[start:end]
 	}
 
-	if n < count || buf[len(buf)-1] != '\n' {
+	if n < count {
 		return n, io.EOF
 	}
 	return n, nil
@@ -75,13 +77,9 @@ func (r *lineReaderAt) ReadLinesAt(lines [][]byte, offset int64) (n int, err err
 // for line or a read returns io.EOF. It returns an error if and only if there
 // is an error reading data.
 func (r *lineReaderAt) indexTo(line int64) error {
-	var buf [1024]byte
-
-	var offset int64
-	if len(r.index) > 0 {
-		offset = r.index[len(r.index)-1]
-	}
+	var buf [indexBufferSize]byte
 
+	offset := r.lastOffset()
 	for int64(len(r.index)) < line {
 		n, err := r.r.ReadAt(buf[:], offset)
 		if err != nil && err != io.EOF {
@@ -94,7 +92,7 @@ func (r *lineReaderAt) indexTo(line int64) error {
 			}
 		}
 		if err == io.EOF {
-			if n > 0 && buf[n-1] != '\n' {
+			if offset > r.lastOffset() {
 				r.index = append(r.index, offset)
 			}
 			r.eof = true
@@ -104,6 +102,13 @@ func (r *lineReaderAt) indexTo(line int64) error {
 	return nil
 }
 
+func (r *lineReaderAt) lastOffset() int64 {
+	if n := len(r.index); n > 0 {
+		return r.index[n-1]
+	}
+	return 0
+}
+
 // readBytes reads the bytes of the n lines starting at line and returns the
 // bytes and the offset of the first byte in the underlying source.
 func (r *lineReaderAt) readBytes(line, n int64) (b []byte, offset int64, err error) {
@@ -147,11 +152,6 @@ func isLen(r io.ReaderAt, n int64) (bool, error) {
 	return false, err
 }
 
-const (
-	byteBufferSize = 32 * 1024 // from io.Copy
-	lineBufferSize = 32
-)
-
 // copyFrom writes bytes starting from offset off in src to dst stopping at the
 // end of src or at the first error. copyFrom returns the number of bytes
 // written and any error.
diff --git a/gitdiff/io_test.go b/gitdiff/io_test.go
index 8d8a18b..bd242a7 100644
--- a/gitdiff/io_test.go
+++ b/gitdiff/io_test.go
@@ -9,6 +9,8 @@ import (
 )
 
 func TestLineReaderAt(t *testing.T) {
+	const lineTemplate = "generated test line %d\n"
+
 	tests := map[string]struct {
 		InputLines int
 		Offset     int64
@@ -42,6 +44,11 @@ func TestLineReaderAt(t *testing.T) {
 			Offset:     2,
 			Count:      0,
 		},
+		"readAllLines": {
+			InputLines: 64,
+			Offset:     0,
+			Count:      64,
+		},
 		"readThroughEOF": {
 			InputLines: 16,
 			Offset:     12,
@@ -71,8 +78,6 @@ func TestLineReaderAt(t *testing.T) {
 		},
 	}
 
-	const lineTemplate = "generated test line %d\n"
-
 	for name, test := range tests {
 		t.Run(name, func(t *testing.T) {
 			var input bytes.Buffer
@@ -114,6 +119,53 @@ func TestLineReaderAt(t *testing.T) {
 			}
 		})
 	}
+
+	newlineTests := map[string]struct {
+		InputSize int
+	}{
+		"readLinesNoFinalNewline": {
+			InputSize: indexBufferSize + indexBufferSize/2,
+		},
+		"readLinesNoFinalNewlineBufferMultiple": {
+			InputSize: 4 * indexBufferSize,
+		},
+	}
+
+	for name, test := range newlineTests {
+		t.Run(name, func(t *testing.T) {
+			input := bytes.Repeat([]byte("0"), test.InputSize)
+
+			var output [][]byte
+			for i := 0; i < len(input); i++ {
+				last := i
+				i += rand.Intn(80)
+				if i < len(input)-1 { // last character of input must not be a newline
+					input[i] = '\n'
+					output = append(output, input[last:i+1])
+				} else {
+					output = append(output, input[last:])
+				}
+			}
+
+			r := &lineReaderAt{r: bytes.NewReader(input)}
+			lines := make([][]byte, len(output))
+
+			n, err := r.ReadLinesAt(lines, 0)
+			if err != nil {
+				t.Fatalf("unexpected error reading reading lines: %v", err)
+			}
+
+			if n != len(output) {
+				t.Fatalf("incorrect number of lines read: expected %d, actual %d", len(output), n)
+			}
+
+			for i, line := range lines {
+				if !bytes.Equal(output[i], line) {
+					t.Errorf("incorrect content in line %d:\nexpected: %q\nactual: %q", i, output[i], line)
+				}
+			}
+		})
+	}
 }
 
 func TestCopyFrom(t *testing.T) {