diff --git a/decode.go b/decode.go
index 0ac8e4f..4ca5ee3 100644
--- a/decode.go
+++ b/decode.go
@@ -121,7 +121,7 @@
 			return nil, errUnsupportedCopy4Tag
 		}
 
-		if offset > d || length > len(dst)-d {
+		if offset <= 0 || d < offset || length > len(dst)-d {
 			return nil, ErrCorrupt
 		}
 		for end := d + length; d != end; d++ {
diff --git a/snappy_test.go b/snappy_test.go
index 4d77669..a6fbbf0 100644
--- a/snappy_test.go
+++ b/snappy_test.go
@@ -198,6 +198,11 @@
 		"abcdabcd",
 		nil,
 	}, {
+		`decodedLen=8; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=0; zero offset`,
+		"\x08" + "\x0cabcd" + "\x01\x00",
+		"",
+		ErrCorrupt,
+	}, {
 		`decodedLen=9; tagLiteral (4 bytes "abcd"); tagCopy1; length=4 offset=4; inconsistent dLen`,
 		"\x09" + "\x0cabcd" + "\x01\x04",
 		"",