diff --git a/mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/ExternalImage.kt b/mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/ExternalImage.kt
index ef084b7b2..cc183a496 100644
--- a/mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/ExternalImage.kt
+++ b/mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/ExternalImage.kt
@@ -183,8 +183,8 @@ internal operator fun ByteArray.get(range: IntRange): String = buildString {
 }
 
 private fun Byte.fixToString(): String {
-    return when (this.toInt()) {
+    return when (val b = this.toInt() and 0xff) {
         in 0..15 -> "0${this.toString(16).toUpperCase()}"
-        else -> this.toString(16).toUpperCase()
+        else -> b.toString(16).toUpperCase()
     }
 }
\ No newline at end of file
diff --git a/mirai-core/src/commonTest/kotlin/net/mamoe/mirai/utils/ExternalImageTest.kt b/mirai-core/src/commonTest/kotlin/net/mamoe/mirai/utils/ExternalImageTest.kt
index 6cc178d5f..50cc3b5fd 100644
--- a/mirai-core/src/commonTest/kotlin/net/mamoe/mirai/utils/ExternalImageTest.kt
+++ b/mirai-core/src/commonTest/kotlin/net/mamoe/mirai/utils/ExternalImageTest.kt
@@ -9,6 +9,6 @@ internal class ExternalImageTest {
     fun testByteArrayGet() {
         assertEquals("0F", byteArrayOf(0x0f)[0..0])
         assertEquals("10", byteArrayOf(0x10)[0..0])
-        assertEquals("0FFE", byteArrayOf(0x0F, 0xFE.toByte())[0..0])
+        assertEquals("0FFE", byteArrayOf(0x0F, 0xFE.toByte())[0..1])
     }
 }
\ No newline at end of file