diff --git a/internal/rtmp/rtmp.go b/internal/rtmp/rtmp.go
index 2eaa12cbe..29826f46b 100644
--- a/internal/rtmp/rtmp.go
+++ b/internal/rtmp/rtmp.go
@@ -714,6 +714,11 @@ func (v *protocol) readBasicHeader(ctx context.Context) (format formatType, cid
 		return
 	}
 
+	// Here cid is 0 or 1: a marker selecting the 2B or 3B form, not the real cid. Keep it,
+	// because cid is overwritten below and the marker decides whether a third byte (the
+	// high-order part of the cid) follows. Do not test the overwritten cid for this.
+	marker := cid
+
 	// 64-319, 2B chunk header
 	if err = binary.Read(v.r, binary.BigEndian, &t); err != nil {
 		return format, cid, errors.Wrapf(err, "read basic header for cid=%v", cid)
@@ -721,7 +726,7 @@ func (v *protocol) readBasicHeader(ctx context.Context) (format formatType, cid
 	cid = chunkID(64 + uint32(t))
 
 	// 64-65599, 3B chunk header
-	if cid == 1 {
+	if marker == 1 {
 		if err = binary.Read(v.r, binary.BigEndian, &t); err != nil {
 			return format, cid, errors.Wrapf(err, "read basic header for cid=%v", cid)
 		}
diff --git a/internal/rtmp/rtmp_test.go b/internal/rtmp/rtmp_test.go
index 09dc4c5f4..b23268842 100644
--- a/internal/rtmp/rtmp_test.go
+++ b/internal/rtmp/rtmp_test.go
@@ -98,7 +98,7 @@ func TestBasicHeaderVariantsAndErrors(t *testing.T) {
 	}{
 		{"one-byte", []byte{0x85}, formatType2, 5},
 		{"two-byte", []byte{0x40, 0x0a}, formatType1, 74},
-		{"three-byte-code-path", []byte{0xc1, 0x01, 0x02}, formatType3, 65},
+		{"three-byte", []byte{0xc1, 0x01, 0x02}, formatType3, 577},
 	}
 	for _, tt := range cases {
 		t.Run(tt.name, func(t *testing.T) {