diff --git a/src/pkg/image/png/reader.go b/src/pkg/image/png/reader.go
index 8c76afa72c6..aa023741d07 100644
--- a/src/pkg/image/png/reader.go
+++ b/src/pkg/image/png/reader.go
@@ -489,6 +489,16 @@ func (d *decoder) idatReader(idat io.Reader) (image.Image, os.Error) {
 		// The current row for y is the previous row for y+1.
 		pr, cr = cr, pr
 	}
+
+	// Check for EOF, to verify the zlib checksum.
+	n, err := r.Read(pr[:1])
+	if err != os.EOF {
+		return nil, FormatError(err.String())
+	}
+	if n != 0 {
+		return nil, FormatError("too much pixel data")
+	}
+
 	return img, nil
 }
 
diff --git a/src/pkg/image/png/reader_test.go b/src/pkg/image/png/reader_test.go
index bcc1a3db475..20884319058 100644
--- a/src/pkg/image/png/reader_test.go
+++ b/src/pkg/image/png/reader_test.go
@@ -10,6 +10,7 @@ import (
 	"image"
 	"io"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -41,7 +42,7 @@ var filenamesShort = []string{
 	"basn6a16",
 }
 
-func readPng(filename string) (image.Image, os.Error) {
+func readPNG(filename string) (image.Image, os.Error) {
 	f, err := os.Open(filename)
 	if err != nil {
 		return nil, err
@@ -183,7 +184,7 @@ func TestReader(t *testing.T) {
 	}
 	for _, fn := range names {
 		// Read the .png file.
-		img, err := readPng("testdata/pngsuite/" + fn + ".png")
+		img, err := readPNG("testdata/pngsuite/" + fn + ".png")
 		if err != nil {
 			t.Error(fn, err)
 			continue
@@ -239,3 +240,29 @@ func TestReader(t *testing.T) {
 		}
 	}
 }
+
+var readerErrors = []struct {
+	file string
+	err  string
+}{
+	{"invalid-zlib.png", "zlib checksum error"},
+	{"invalid-crc32.png", "invalid checksum"},
+	{"invalid-noend.png", "unexpected EOF"},
+	{"invalid-trunc.png", "unexpected EOF"},
+}
+
+func TestReaderError(t *testing.T) {
+	for _, tt := range readerErrors {
+		img, err := readPNG("testdata/" + tt.file)
+		if err == nil {
+			t.Errorf("decoding %s: missing error", tt.file)
+			continue
+		}
+		if !strings.Contains(err.String(), tt.err) {
+			t.Errorf("decoding %s: %s, want %s", tt.file, err, tt.err)
+		}
+		if img != nil {
+			t.Errorf("decoding %s: have image + error")
+		}
+	}
+}
diff --git a/src/pkg/image/png/testdata/invalid-crc32.png b/src/pkg/image/png/testdata/invalid-crc32.png
new file mode 100644
index 00000000000..e5be4086cb2
Binary files /dev/null and b/src/pkg/image/png/testdata/invalid-crc32.png differ
diff --git a/src/pkg/image/png/testdata/invalid-noend.png b/src/pkg/image/png/testdata/invalid-noend.png
new file mode 100644
index 00000000000..9137270d9c3
Binary files /dev/null and b/src/pkg/image/png/testdata/invalid-noend.png differ
diff --git a/src/pkg/image/png/testdata/invalid-trunc.png b/src/pkg/image/png/testdata/invalid-trunc.png
new file mode 100644
index 00000000000..d0748cf6541
Binary files /dev/null and b/src/pkg/image/png/testdata/invalid-trunc.png differ
diff --git a/src/pkg/image/png/testdata/invalid-zlib.png b/src/pkg/image/png/testdata/invalid-zlib.png
new file mode 100644
index 00000000000..c6d051caee0
Binary files /dev/null and b/src/pkg/image/png/testdata/invalid-zlib.png differ
diff --git a/src/pkg/image/png/writer_test.go b/src/pkg/image/png/writer_test.go
index 1599791b3a3..046aad9d27d 100644
--- a/src/pkg/image/png/writer_test.go
+++ b/src/pkg/image/png/writer_test.go
@@ -56,13 +56,13 @@ func TestWriter(t *testing.T) {
 	for _, fn := range names {
 		qfn := "testdata/pngsuite/" + fn + ".png"
 		// Read the image.
-		m0, err := readPng(qfn)
+		m0, err := readPNG(qfn)
 		if err != nil {
 			t.Error(fn, err)
 			continue
 		}
 		// Read the image again, encode it, and decode it.
-		m1, err := readPng(qfn)
+		m1, err := readPNG(qfn)
 		if err != nil {
 			t.Error(fn, err)
 			return