diff --git a/protocol/core/shell.go b/protocol/core/shell.go
index 4ab9687..f9661fd 100644
--- a/protocol/core/shell.go
+++ b/protocol/core/shell.go
@@ -28,8 +28,8 @@ func GetRoundFromTenderbakeBlock(data tz.Bytes) (uint32, error) {
 	   <predecessor_round_len(4)><predecessor_round(4)>
 	   <round_len(4)><round(4)> */
 
-	if len(data) < 4 {
-		return 0, errors.New("data too short to extract round")
+	if len(data) < 33 {
+		return 0, errors.New("data too short to be valid tenderbake fitness")
 	}
 	// The fitness data has been stripped from its prefixed length
 	// The round value is always the 4 last bytes
diff --git a/protocol/core/shell_test.go b/protocol/core/shell_test.go
new file mode 100644
index 0000000..ea2f11b
--- /dev/null
+++ b/protocol/core/shell_test.go
@@ -0,0 +1,99 @@
+package core
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetRoundFromTenderbakeBlock(t *testing.T) {
+	// Helper to build valid tenderbake fitness bytes.
+	// Format: <version_len(4)><version(1)>
+	//         <level_len(4)><level(4)>
+	//         <locked_round_len(4)><locked_round(0 or 4)>
+	//         <predecessor_round_len(4)><predecessor_round(4)>
+	//         <round_len(4)><round(4)>
+	buildFitness := func(level uint32, lockedRound *uint32, predecessorRound uint32, round uint32) []byte {
+		b := []byte{
+			0, 0, 0, 1, 0x02, // version_len=1, version=2
+			0, 0, 0, 4, byte(level >> 24), byte(level >> 16), byte(level >> 8), byte(level), // level
+		}
+		if lockedRound == nil {
+			b = append(b, 0, 0, 0, 0) // locked_round_len=0
+		} else {
+			lr := *lockedRound
+			b = append(b, 0, 0, 0, 4, byte(lr>>24), byte(lr>>16), byte(lr>>8), byte(lr))
+		}
+		b = append(b, 0, 0, 0, 4, byte(predecessorRound>>24), byte(predecessorRound>>16), byte(predecessorRound>>8), byte(predecessorRound))
+		b = append(b, 0, 0, 0, 4, byte(round>>24), byte(round>>16), byte(round>>8), byte(round))
+		return b
+	}
+
+	uint32Ptr := func(v uint32) *uint32 { return &v }
+
+	t.Run("real mainnet fitness round 0", func(t *testing.T) {
+		// From proto_016_PtMumbai operations_test.go (double_baking_evidence),
+		// mainnet level 3365599. Fitness version 0x02 (Tenderbake, stable since proto_012).
+		fitness := []byte{
+			0x00, 0x00, 0x00, 0x01, 0x02,
+			0x00, 0x00, 0x00, 0x04, 0x00, 0x33, 0x5a, 0xdf,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x04, 0xff, 0xff, 0xff, 0xff,
+			0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00,
+		}
+		round, err := GetRoundFromTenderbakeBlock(fitness)
+		require.NoError(t, err)
+		assert.Equal(t, uint32(0), round)
+	})
+
+	t.Run("real mainnet fitness round 1", func(t *testing.T) {
+		// Mainnet level 12169657 (proto_024), block round 1 (reproposal).
+		// Fitness from RPC: ["02","00b9b1b9","","ffffffff","00000001"]
+		fitness := []byte{
+			0x00, 0x00, 0x00, 0x01, 0x02,
+			0x00, 0x00, 0x00, 0x04, 0x00, 0xb9, 0xb1, 0xb9,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x04, 0xff, 0xff, 0xff, 0xff,
+			0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x01,
+		}
+		round, err := GetRoundFromTenderbakeBlock(fitness)
+		require.NoError(t, err)
+		assert.Equal(t, uint32(1), round)
+	})
+
+	t.Run("round extraction without locked round", func(t *testing.T) {
+		fitness := buildFitness(100, nil, 0, 5)
+		round, err := GetRoundFromTenderbakeBlock(fitness)
+		require.NoError(t, err)
+		assert.Equal(t, uint32(5), round)
+	})
+
+	t.Run("round extraction with locked round", func(t *testing.T) {
+		// With locked round present, fitness is 37 bytes instead of 33
+		fitness := buildFitness(100, uint32Ptr(2), 0, 7)
+		round, err := GetRoundFromTenderbakeBlock(fitness)
+		require.NoError(t, err)
+		assert.Equal(t, uint32(7), round)
+	})
+
+	t.Run("reproposal has different round than original", func(t *testing.T) {
+		// This is the actual scenario that caused the bug: a block reproposed
+		// at a higher round. The payload_round stays at 0 but the fitness
+		// round is 3.
+		fitness := buildFitness(100, uint32Ptr(0), 0, 3)
+		round, err := GetRoundFromTenderbakeBlock(fitness)
+		require.NoError(t, err)
+		assert.Equal(t, uint32(3), round, "reproposal round should come from fitness, not payload_round")
+	})
+
+	t.Run("rejects truncated fitness", func(t *testing.T) {
+		_, err := GetRoundFromTenderbakeBlock([]byte{0, 0, 0, 1, 2})
+		assert.Error(t, err)
+	})
+
+	t.Run("rejects empty fitness", func(t *testing.T) {
+		_, err := GetRoundFromTenderbakeBlock(nil)
+		assert.Error(t, err)
+	})
+}
diff --git a/protocol/proto_022_PsRiotum/sign_request.go b/protocol/proto_022_PsRiotum/sign_request.go
index 6cb1312..d5874a8 100644
--- a/protocol/proto_022_PsRiotum/sign_request.go
+++ b/protocol/proto_022_PsRiotum/sign_request.go
@@ -59,7 +59,7 @@ func (r *BlockSignRequest) GetLevel() int32         { return r.BlockHeader.Level
 func (r *BlockSignRequest) GetRound() int32 {
 	round, err := core.GetRoundFromTenderbakeBlock(r.BlockHeader.Fitness)
 	if err != nil {
-		fmt.Println("Error: ", err)
+		panic(fmt.Sprintf("BlockSignRequest.GetRound: %v", err))
 	}
 	return int32(round)
 }