diff --git a/internal/ethereum/deploy_contract_prepare.go b/internal/ethereum/deploy_contract_prepare.go
index dea9766..34864d7 100644
--- a/internal/ethereum/deploy_contract_prepare.go
+++ b/internal/ethereum/deploy_contract_prepare.go
@@ -34,7 +34,7 @@ import (
 func (c *ethConnector) DeployContractPrepare(ctx context.Context, req *ffcapi.ContractDeployPrepareRequest) (res *ffcapi.TransactionPrepareResponse, reason ffcapi.ErrorReason, err error) {
 
 	// Parse the input JSON data, to build the call data
-	callData, method, err := c.prepareDeployData(ctx, req)
+	callData, constructor, err := c.prepareDeployData(ctx, req)
 	if err != nil {
 		return nil, ffcapi.ErrorReasonInvalidInputs, err
 	}
@@ -45,10 +45,10 @@ func (c *ethConnector) DeployContractPrepare(ctx context.Context, req *ffcapi.Co
 		return nil, ffcapi.ErrorReasonInvalidInputs, err
 	}
 
-	if req.Gas, reason, err = c.ensureGasEstimate(ctx, tx, method, req.Gas); err != nil {
+	if req.Gas, reason, err = c.ensureGasEstimate(ctx, tx, constructor, req.Gas); err != nil {
 		return nil, reason, err
 	}
-	log.L(ctx).Infof("Prepared transaction method=%s dataLen=%d gas=%s", method.String(), len(callData), req.Gas.Int())
+	log.L(ctx).Infof("Prepared deploy transaction dataLen=%d gas=%s", len(callData), req.Gas.Int())
 
 	return &ffcapi.TransactionPrepareResponse{
 		Gas:             req.Gas,
@@ -100,10 +100,9 @@ func (c *ethConnector) prepareDeployData(ctx context.Context, req *ffcapi.Contra
 	// Note the FireFly ABI decoding package handles formatting errors / translation etc.
 	var callData []byte
 	paramValues, err := method.Inputs.ParseExternalDataCtx(ctx, ethParams)
-	if err != nil {
-		return nil, nil, err
+	if err == nil {
+		callData, err = paramValues.EncodeABIData()
 	}
-	callData, err = paramValues.EncodeABIData()
 	if err != nil {
 		return nil, nil, err
 	}
diff --git a/internal/ethereum/deploy_contract_prepare_test.go b/internal/ethereum/deploy_contract_prepare_test.go
index a788633..4fa593a 100644
--- a/internal/ethereum/deploy_contract_prepare_test.go
+++ b/internal/ethereum/deploy_contract_prepare_test.go
@@ -21,6 +21,7 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/hyperledger/firefly-common/pkg/fftypes"
 	"github.com/hyperledger/firefly-signer/pkg/ethtypes"
 	"github.com/hyperledger/firefly-transaction-manager/pkg/ffcapi"
 	"github.com/stretchr/testify/assert"
@@ -91,4 +92,149 @@ func TestDeployContractPrepareWithEstimateRevert(t *testing.T) {
 	assert.Equal(t, ffcapi.ErrorReasonTransactionReverted, reason)
 	assert.Nil(t, res)
 
+	mRPC.AssertExpectations(t)
+
+}
+
+func TestDeployContractPrepareBadBytecode(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.Contract = fftypes.JSONAnyPtr(`! not a string containing bytecode`)
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF23047", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
+}
+
+func TestDeployContractPrepareBadBytecodeNotHex(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.Contract = fftypes.JSONAnyPtr(`"!hex"`)
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF23047", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
+}
+
+func TestDeployContractPrepareBadABIDefinition(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.Definition = fftypes.JSONAnyPtr(`[`)
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF23013", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
+}
+
+func TestDeployContractPrepareEstimateNoConstructor(t *testing.T) {
+
+	ctx, c, mRPC, done := newTestConnector(t)
+	defer done()
+
+	mRPC.On("Invoke", mock.Anything, mock.Anything, "eth_estimateGas", mock.Anything).Return(nil).Run(func(args mock.Arguments) {
+		*(args[1].(*ethtypes.HexInteger)) = *ethtypes.NewHexInteger64(12345)
+	})
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	assert.NoError(t, err)
+	req.Definition = fftypes.JSONAnyPtr(`[]`)
+	req.Gas = nil
+	res, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.NoError(t, err)
+	assert.Empty(t, reason)
+
+	fGasEstimate, _ := c.gasEstimationFactor.Float64()
+	assert.Equal(t, int64(float64(12345)*fGasEstimate), res.Gas.Int64())
+
+	mRPC.AssertExpectations(t)
+
+}
+
+func TestDeployContractPrepareBadParamsJSON(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.Params = []*fftypes.JSONAny{fftypes.JSONAnyPtr(`"!wrong`)}
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF23014", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
+}
+
+func TestDeployContractPrepareBadParamType(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.Params = []*fftypes.JSONAny{fftypes.JSONAnyPtr(`"!wrong"`)}
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF22030", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
+}
+
+func TestDeployContractPrepareBadFrom(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.From = "!not an address"
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF23019", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
+}
+
+func TestDeployContractPrepareBadABIType(t *testing.T) {
+
+	ctx, c, _, done := newTestConnector(t)
+	defer done()
+
+	var req ffcapi.ContractDeployPrepareRequest
+	err := json.Unmarshal([]byte(samplePrepareDeployTX), &req)
+	req.Definition = fftypes.JSONAnyPtr(`[{
+		"type": "constructor",
+		"inputs": [{
+			"type": "!wrong"
+		}]
+	}]`)
+	assert.NoError(t, err)
+	_, reason, err := c.DeployContractPrepare(ctx, &req)
+
+	assert.Regexp(t, "FF22025", err)
+	assert.Equal(t, ffcapi.ErrorReasonInvalidInputs, reason)
+
 }