diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6bdc3d1f5..4426353d9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,20 @@
 
 This document outlines major changes between releases.
 
+## 0.98.4 "Mesmerization" (11 May 2022)
+
+An urgent release to fix incompatibility with mainnet at block 1528989. The
+actual pair of problems leading to inability to process this block occurred
+earilier than that, so to fix this you need to resynchronize your node. Fixed
+node is confirmed to have identical state as 3.1.0 C# node up to block
+1529810.
+
+Bugs fixed:
+ * StdLib itoa() implementation emitted uppercase letters instead of lowercase
+   (#2478)
+ * StdLib jsonDeserialize() implementation couldn't handle properly integers
+   larger than 64-bit signed (#2478)
+
 ## 0.98.3 "Liquidation" (07 May 2022)
 
 This is a hotfix release to fix t4 testnet incompatibility at block
diff --git a/pkg/core/native/std.go b/pkg/core/native/std.go
index eecd58bf2..00ff22562 100644
--- a/pkg/core/native/std.go
+++ b/pkg/core/native/std.go
@@ -238,7 +238,6 @@ func (s *Std) itoa(_ *interop.Context, args []stackitem.Item) stackitem.Item {
 		if pad := bs[0] & 0xF8; pad == 0 || pad == 0xF8 {
 			str = str[1:]
 		}
-		str = strings.ToUpper(str)
 	default:
 		panic(ErrInvalidBase)
 	}
diff --git a/pkg/core/native/std_test.go b/pkg/core/native/std_test.go
index 2817850e0..da56a4f90 100644
--- a/pkg/core/native/std_test.go
+++ b/pkg/core/native/std_test.go
@@ -35,9 +35,9 @@ func TestStdLibItoaAtoi(t *testing.T) {
 			{big.NewInt(1), big.NewInt(16), "1"},
 			{big.NewInt(7), big.NewInt(16), "7"},
 			{big.NewInt(8), big.NewInt(16), "08"},
-			{big.NewInt(65535), big.NewInt(16), "0FFFF"},
-			{big.NewInt(15), big.NewInt(16), "0F"},
-			{big.NewInt(-1), big.NewInt(16), "F"},
+			{big.NewInt(65535), big.NewInt(16), "0ffff"},
+			{big.NewInt(15), big.NewInt(16), "0f"},
+			{big.NewInt(-1), big.NewInt(16), "f"},
 		}
 
 		for _, tc := range testCases {
diff --git a/pkg/vm/stackitem/json.go b/pkg/vm/stackitem/json.go
index 12e9df816..89540b51d 100644
--- a/pkg/vm/stackitem/json.go
+++ b/pkg/vm/stackitem/json.go
@@ -7,9 +7,9 @@ import (
 	"errors"
 	"fmt"
 	gio "io"
-	"math"
 	"math/big"
 	"strconv"
+	"strings"
 )
 
 // decoder is a wrapper around json.Decoder helping to mimic C# json decoder behavior.
@@ -164,6 +164,7 @@ func FromJSON(data []byte, maxCount int) (Item, error) {
 		Decoder: *json.NewDecoder(bytes.NewReader(data)),
 		count:   maxCount,
 	}
+	d.UseNumber()
 	if item, err := d.decode(); err != nil {
 		return nil, err
 	} else if _, err := d.Token(); err != gio.EOF {
@@ -208,11 +209,24 @@ func (d *decoder) decode() (Item, error) {
 		}
 	case string:
 		return NewByteArray([]byte(t)), nil
-	case float64:
-		if math.Floor(t) != t {
-			return nil, fmt.Errorf("%w (real value for int)", ErrInvalidValue)
+	case json.Number:
+		ts := t.String()
+		dot := strings.IndexByte(ts, '.')
+		if dot != -1 {
+			// As a special case numbers like 123.000 are allowed (SetString rejects them).
+			// And yes, that's the way C# code works also.
+			for _, r := range ts[dot+1:] {
+				if r != '0' {
+					return nil, fmt.Errorf("%w (real value for int)", ErrInvalidValue)
+				}
+			}
+			ts = ts[:dot]
 		}
-		return NewBigInteger(big.NewInt(int64(t))), nil
+		num, ok := new(big.Int).SetString(ts, 10)
+		if !ok {
+			return nil, fmt.Errorf("%w (integer)", ErrInvalidValue)
+		}
+		return NewBigInteger(num), nil
 	case bool:
 		return NewBool(t), nil
 	default: