diff --git a/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt b/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt
index 7ce0f709c..fc9a5236c 100644
--- a/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt
+++ b/formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt
@@ -248,7 +248,7 @@ private class DynamicMapInput(
         if (isKey) {
             val value = decodeTaggedValue(tag)
             if (value !is String) return decode(tag)
-            return value.toString().cast() ?: throwIllegalKeyType(tag, value, type)
+            return value.cast() ?: throwIllegalKeyType(tag, value, type)
         }
         return decode(tag)
     }