diff --git a/zstd/blockdec.go b/zstd/blockdec.go
index 9f17ce601f..03744fbc76 100644
--- a/zstd/blockdec.go
+++ b/zstd/blockdec.go
@@ -554,6 +554,9 @@ func (b *blockDec) prepareSequences(in []byte, hist *history) (err error) {
 		if debugDecoder {
 			printf("Compression modes: 0b%b", compMode)
 		}
+		if compMode&3 != 0 {
+			return errors.New("corrupt block: reserved bits not zero")
+		}
 		for i := uint(0); i < 3; i++ {
 			mode := seqCompMode((compMode >> (6 - i*2)) & 3)
 			if debugDecoder {