diff --git a/src/image/png/reader.go b/src/image/png/reader.go
index 910520b..4c65038 100644
--- a/src/image/png/reader.go
+++ b/src/image/png/reader.go
@@ -821,9 +821,17 @@
 		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
 		bytesPerPixel = 8
 	case *image.Paletted:
-		srcPix = src.(*image.Paletted).Pix
+		source := src.(*image.Paletted)
+		srcPix = source.Pix
 		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
 		bytesPerPixel = 1
+		if len(target.Palette) < len(source.Palette) {
+			// readImagePass can return a paletted image whose implicit palette
+			// length (one more than the maximum Pix value) is larger than the
+			// explicit palette length (what's in the PLTE chunk). Make the
+			// same adjustment here.
+			target.Palette = source.Palette
+		}
 	case *image.RGBA:
 		srcPix = src.(*image.RGBA).Pix
 		dstPix, stride, rect = target.Pix, target.Stride, target.Rect