diff --git a/rpc/client/args_test.go b/rpc/client/args_test.go
new file mode 100644
index 000000000..ccabd0d2c
--- /dev/null
+++ b/rpc/client/args_test.go
@@ -0,0 +1,39 @@
+package rpcclient
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type Tx []byte
+
+type Foo struct {
+	Bar int
+	Baz string
+}
+
+func TestArgToJSON(t *testing.T) {
+	assert := assert.New(t)
+	require := require.New(t)
+
+	cases := []struct {
+		input    interface{}
+		expected string
+	}{
+		{[]byte("1234"), "0x31323334"},
+		{Tx("654"), "0x363534"},
+		{Foo{7, "hello"}, `{"Bar":7,"Baz":"hello"}`},
+	}
+
+	for i, tc := range cases {
+		args := map[string]interface{}{"data": tc.input}
+		err := argsToJson(args)
+		require.Nil(err, "%d: %+v", i, err)
+		require.Equal(1, len(args), "%d", i)
+		data, ok := args["data"].(string)
+		require.True(ok, "%d: %#v", i, args["data"])
+		assert.Equal(tc.expected, data, "%d", i)
+	}
+}
diff --git a/rpc/client/http_client.go b/rpc/client/http_client.go
index ce78da43d..38a7f20fc 100644
--- a/rpc/client/http_client.go
+++ b/rpc/client/http_client.go
@@ -12,8 +12,8 @@ import (
 	"strings"
 
 	"github.com/pkg/errors"
-	types "github.com/tendermint/tendermint/rpc/types"
 	wire "github.com/tendermint/go-wire"
+	types "github.com/tendermint/tendermint/rpc/types"
 )
 
 // HTTPClient is a common interface for JSONRPCClient and URIClient.
@@ -179,10 +179,11 @@ func argsToJson(args map[string]interface{}) error {
 	var n int
 	var err error
 	for k, v := range args {
-		// Convert byte slices to "0x"-prefixed hex
-		byteSlice, isByteSlice := reflect.ValueOf(v).Interface().([]byte)
+		rt := reflect.TypeOf(v)
+		isByteSlice := rt.Kind() == reflect.Slice && rt.Elem().Kind() == reflect.Uint8
 		if isByteSlice {
-			args[k] = fmt.Sprintf("0x%X", byteSlice)
+			bytes := reflect.ValueOf(v).Bytes()
+			args[k] = fmt.Sprintf("0x%X", bytes)
 			continue
 		}