diff --git a/lib/rac/reader.go b/lib/rac/reader.go
index 503809a..4e9710e 100644
--- a/lib/rac/reader.go
+++ b/lib/rac/reader.go
@@ -95,7 +95,10 @@
 	// Bigger values often lead to faster throughput, up to a
 	// hardware-dependent point, but also larger memory requirements.
 	//
-	// Zero means a non-concurrent (single-goroutine) reader.
+	// If positive, then the ReadSeeker must also be an io.ReaderAt.
+	//
+	// Non-positive values (including zero) mean a non-concurrent
+	// (single-goroutine) reader.
 	Concurrency int
 
 	// err is the first error encountered. It is sticky: once a non-nil error
@@ -175,6 +178,12 @@
 	}
 	r.chunkReader.ReadSeeker = r.ReadSeeker
 	r.chunkReader.CompressedSize = r.CompressedSize
+	if r.Concurrency > 0 {
+		if _, ok := r.ReadSeeker.(io.ReaderAt); !ok {
+			r.err = fmt.Errorf("rac: Concurrency > 0 requires the ReadSeeker to be an io.ReaderAt")
+			return r.err
+		}
+	}
 	if err := r.chunkReader.initialize(); err != nil {
 		r.err = err
 		return r.err
diff --git a/lib/raczlib/raczlib_test.go b/lib/raczlib/raczlib_test.go
index 318f5c2..0bcee96 100644
--- a/lib/raczlib/raczlib_test.go
+++ b/lib/raczlib/raczlib_test.go
@@ -75,12 +75,13 @@
 	return buf.Bytes(), nil
 }
 
-func racDecompress(compressed []byte) ([]byte, error) {
+func racDecompress(compressed []byte, concurrency int) ([]byte, error) {
 	buf := &bytes.Buffer{}
 	r := &rac.Reader{
 		ReadSeeker:     bytes.NewReader(compressed),
 		CompressedSize: int64(len(compressed)),
 		CodecReaders:   []rac.CodecReader{&CodecReader{}},
+		Concurrency:    concurrency,
 	}
 	defer r.Close()
 	if _, err := io.Copy(buf, r); err != nil {
@@ -89,8 +90,8 @@
 	return buf.Bytes(), nil
 }
 
-func testReader(tt *testing.T, decoded string, encoded string) {
-	g, err := racDecompress([]byte(encoded))
+func testReader(tt *testing.T, decoded string, encoded string, concurrency int) {
+	g, err := racDecompress([]byte(encoded), concurrency)
 	if err != nil {
 		tt.Fatalf("racDecompress: %v", err)
 	}
@@ -99,8 +100,9 @@
 	}
 }
 
-func TestReaderSansDictionary(tt *testing.T) { testReader(tt, decodedMore, encodedMore) }
-func TestReaderWithDictionary(tt *testing.T) { testReader(tt, decodedSheep, encodedSheep) }
+func TestReaderSansDictionary(tt *testing.T) { testReader(tt, decodedMore, encodedMore, 0) }
+func TestReaderWithDictionary(tt *testing.T) { testReader(tt, decodedSheep, encodedSheep, 0) }
+func TestConcurrentReader(tt *testing.T)     { testReader(tt, decodedSheep, encodedSheep, 2) }
 
 func TestReaderConcatenation(tt *testing.T) {
 	// Create a RAC file whose decoding is the concatenation of two other RAC
@@ -174,6 +176,7 @@
 	testReader(tt,
 		decodedSheep+decodedMore,
 		encodedSheep+encodedMore+string(buf[:]),
+		0,
 	)
 }
 
@@ -262,7 +265,7 @@
 		compressedLengths[i] = len(compressed)
 
 		// Decompress.
-		decompressed, err := racDecompress(compressed)
+		decompressed, err := racDecompress(compressed, 0)
 		if err != nil {
 			tt.Fatalf("i=%d: racDecompress: %v", i, err)
 		}