MuleSoft DataWeave Errors: Common Issues and How to Fix Them

DataWeave is powerful but produces cryptic errors. This guide decodes the most common DataWeave errors and provides tested solutions.

Error: Cannot Coerce :null to :object

Symptom:

Cannot coerce Null (null) to Object
org.mule.weave.v2.exception.MuleWeaveException: Cannot coerce Null (null) to Object

Cause: Accessing properties on a null value or null payload.

Solution 1 - Null-safe navigation:

%dw 2.0
output application/json
---
// ❌ Fails if payload is null
{
    name: payload.customer.name
}
 
// ✅ Null-safe with default
{
    name: payload.customer.name default "Unknown"
}
 
// ✅ Full null-safe chain
{
    name: (payload.customer default {}).name default "Unknown"
}
 
// ✅ Using unless-otherwise
{
    name: payload.customer.name unless payload.customer == null otherwise "Unknown"
}

Solution 2 - Conditional check:

%dw 2.0
output application/json
---
if (payload != null and payload.customer != null)
    { name: payload.customer.name }
else
    { name: "N/A" }

Error: Cannot Coerce :string to :number

Symptom:

Cannot coerce String "abc" to Number
You called the function '+' with these arguments: String, Number

Cause: Performing numeric operations on string values.

Solution - Explicit type conversion:

%dw 2.0
output application/json
---
{
    // ❌ Fails if quantity is string "10"
    total: payload.quantity * payload.price,
 
    // ✅ Convert to number first
    total: (payload.quantity as Number default 0) * (payload.price as Number default 0),
 
    // ✅ With validation
    total: if (payload.quantity is Number and payload.price is Number)
               payload.quantity * payload.price
           else
               0
}

Safe numeric conversion function:

%dw 2.0
output application/json
 
fun safeNumber(value) =
    if (value is Number) value
    else if (value is String and value matches /^-?\d+\.?\d*$/)
        value as Number
    else 0
---
{
    amount: safeNumber(payload.amount)
}

Error: Unable to Parse JSON/XML

Symptom:

Unable to read JSON at 1:1
Exception while executing parse: Expected a json value
Unexpected character '<' at index 0

Cause: Content type mismatch or malformed data.

Solution 1 - Force correct MIME type:

%dw 2.0
output application/json
---
// When payload is string containing JSON
read(payload, "application/json")
 
// When payload is string containing XML
read(payload, "application/xml")

Solution 2 - Handle mixed content:

%dw 2.0
output application/json
 
fun tryParseJson(content) =
    try(read(content, "application/json")) orElse null
 
fun tryParseXml(content) =
    try(read(content, "application/xml")) orElse null
---
{
    data: tryParseJson(payload) default tryParseXml(payload) default payload
}

Solution 3 - Clean BOM and whitespace:

%dw 2.0
output application/json
---
// Remove BOM and leading/trailing whitespace
read(trim(payload replace /^\uFEFF/ with ""), "application/json")

Error: Scripting Error - Heap Space

Symptom:

java.lang.OutOfMemoryError: Java heap space
org.mule.weave.v2.exception.MuleWeaveException: Max output size exceeded

Cause: Processing large payloads or inefficient transformations.

Solution 1 - Enable streaming:

%dw 2.0
output application/json deferred=true
---
payload map ((item) -> {
    id: item.id,
    name: item.name
})

Solution 2 - Use streaming in Mule config:

<ee:transform doc:name="Transform">
    <ee:message>
        <ee:set-payload>
            <![CDATA[
                %dw 2.0
                output application/json streaming=true
                ---
                payload
            ]]>
        </ee:set-payload>
    </ee:message>
</ee:transform>

Solution 3 - Batch processing pattern:

%dw 2.0
output application/json
 
var batchSize = 1000
var totalBatches = ceil(sizeOf(payload) / batchSize)
---
(1 to totalBatches) map ((batchNum) ->
    payload[(batchNum - 1) * batchSize to batchNum * batchSize - 1]
)

Error: Key Already Present

Symptom:

Duplicate key 'name' found at selector 'name'

Cause: Object has duplicate keys (common when merging).

Solution - Handle duplicates explicitly:

%dw 2.0
output application/json
 
// Merge objects, last value wins
fun mergeObjects(obj1, obj2) =
    obj1 ++ obj2
 
// Merge with custom conflict resolution
fun mergeWithPriority(obj1, obj2) =
    (obj1 -- keysOf(obj2)) ++ obj2
---
{
    result: mergeWithPriority(payload.defaults, payload.overrides)
}

Error: Index Out of Bounds

Symptom:

Index 5 out of bounds for length 3
java.lang.ArrayIndexOutOfBoundsException

Cause: Accessing array index that doesn't exist.

Solution - Safe array access:

%dw 2.0
output application/json
---
{
    // ❌ Fails if array has less than 3 items
    thirdItem: payload.items[2],
 
    // ✅ Safe access with default
    thirdItem: payload.items[2] default null,
 
    // ✅ Check length first
    thirdItem: if (sizeOf(payload.items default []) > 2)
                   payload.items[2]
               else
                   null,
 
    // ✅ Safe first/last
    firstItem: payload.items[0] default null,
    lastItem: payload.items[-1] default null
}

Error: Date Parsing Failed

Symptom:

Unable to parse '2025-13-45' as Date
Text '2025/01/15' could not be parsed at index 4

Cause: Date format mismatch or invalid date.

Solution - Flexible date parsing:

%dw 2.0
output application/json
 
fun parseFlexibleDate(dateStr) =
    if (dateStr == null) null
    else if (dateStr matches /^\d{4}-\d{2}-\d{2}$/)
        dateStr as Date {format: "yyyy-MM-dd"}
    else if (dateStr matches /^\d{2}\/\d{2}\/\d{4}$/)
        dateStr as Date {format: "MM/dd/yyyy"}
    else if (dateStr matches /^\d{4}\/\d{2}\/\d{2}$/)
        dateStr as Date {format: "yyyy/MM/dd"}
    else
        try(dateStr as Date) orElse null
 
fun safeDateFormat(date, format) =
    if (date != null)
        date as String {format: format}
    else
        null
---
{
    parsedDate: parseFlexibleDate(payload.dateString),
    formatted: safeDateFormat(parseFlexibleDate(payload.dateString), "yyyy-MM-dd")
}

Error: Encoding Issues (UTF-8)

Symptom:

MalformedInputException: Input length = 1
Unmappable character for encoding UTF-8

Cause: Invalid characters or wrong encoding.

Solution - Force encoding:

%dw 2.0
output application/json encoding="UTF-8"
---
// Clean problematic characters
payload.text replace /[\x00-\x08\x0B\x0C\x0E-\x1F]/ with ""

In Mule flow - set encoding:

<set-payload value="#[payload]" mimeType="application/json; charset=UTF-8"/>

Error: Maximum Call Stack Exceeded

Symptom:

Stack overflow
Maximum call stack size exceeded in recursive function

Cause: Infinite recursion or deep nesting.

Solution - Tail recursion or iteration:

%dw 2.0
output application/json
 
// ❌ Regular recursion (can overflow)
fun flattenBad(arr) =
    arr flatMap ((item) ->
        if (item is Array) flattenBad(item)
        else [item]
    )
 
// ✅ Use built-in flatten with depth limit
fun flattenSafe(arr, maxDepth = 10) =
    if (maxDepth <= 0) arr
    else flatten(arr map ((item) ->
        if (item is Array) flattenSafe(item, maxDepth - 1)
        else [item]
    ))
---
{
    flattened: flattenSafe(payload.nestedArray)
}

DataWeave Debugging Techniques

Log intermediate values:

%dw 2.0
output application/json
import * from dw::Runtime
 
var step1 = payload.data
var logged1 = log("Step 1", step1)
var step2 = step1 filter ($.active == true)
var logged2 = log("Step 2", step2)
---
step2

Try-catch pattern:

%dw 2.0
output application/json
---
{
    result: try(() -> riskyOperation(payload))
        .success
        default "Operation failed",
 
    error: try(() -> riskyOperation(payload))
        .error.message
        default null
}

Type inspection:

%dw 2.0
output application/json
---
{
    payloadType: typeOf(payload),
    isArray: payload is Array,
    isObject: payload is Object,
    keys: if (payload is Object) keysOf(payload) else null,
    size: sizeOf(payload default [])
}

Quick Reference: Common Fixes

| Error | Quick Fix | |-------|-----------| | Cannot coerce null | Add default value | | Cannot coerce string to number | Use as Number default 0 | | Unable to parse JSON | Use read(payload, "application/json") | | Heap space | Add streaming=true or deferred=true | | Duplicate key | Use -- to remove before merge | | Index out of bounds | Use [n] default null | | Date parse error | Use explicit format pattern | | Encoding error | Set encoding="UTF-8" |

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MuleSoft DataWeave Errors: Common Issues and How to Fix Them

Error: Cannot Coerce :null to :object

Error: Cannot Coerce :string to :number

Error: Unable to Parse JSON/XML

Error: Scripting Error - Heap Space

Error: Key Already Present

Error: Index Out of Bounds

Error: Date Parsing Failed

Error: Encoding Issues (UTF-8)

Error: Maximum Call Stack Exceeded

DataWeave Debugging Techniques

Log intermediate values:

Try-catch pattern:

Type inspection:

Quick Reference: Common Fixes

Complex DataWeave Transformations?

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