Camunda DMN Boolean Error in Production – Root Cause and Fix

 One of the most common Camunda DMN issues seen in production environments is the infamous Boolean evaluation error, where a decision table fails with a message indicating that a condition did not return a Boolean value.

This problem often:

• Does not appear in development

• Surfaces suddenly after deployment

• Blocks process execution in production

In this blog, we’ll cover:

• The exact root causes of this issue

• Why it appears unexpectedly in production

• Proven fixes used in real projects

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🔴 Typical Error Message

You may see an error similar to:

Cannot evaluate expression: condition expression returns non-Boolean:
result has class java.lang.String and not java.lang.Boolean

or

FEEL expression did not return a Boolean result

This usually occurs when:

• A process reaches a DMN decision task

• A decision table condition is evaluated

• Camunda expects true or false, but receives something else

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🧠 Root Cause Analysis (Most Important Section)

1️⃣ Using "true" / "false" as Strings Instead of Boolean

This is the #1 root cause.

❌ Incorrect DMN condition:

= "true"

or

= "false"

Camunda interprets this as a String, not a Boolean.

✔ Correct condition:

= true

or simply:

true

📌 DMN conditions must always evaluate to a Boolean value.

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2️⃣ Input Variable Type Mismatch

In many production issues:

• The DMN expects a Boolean

• But the process variable is actually:

  • A String

  • Or comes from REST / form input as "true"

Example:

{
  "approved": "true"
}

Even though it looks correct, this is a String, not a Boolean.

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3️⃣ REST API / Form Submissions Converting Booleans to Strings

This commonly happens when:

• Variables are passed via REST

• Forms submit values as strings

• External systems send "true" / "false"

The DMN engine does not auto-convert strings to Boolean.

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4️⃣ FEEL Expression Returning Non-Boolean

Example of problematic FEEL expression:

approved = "true"

This returns a String comparison, not a Boolean.

Correct FEEL expression:

approved = true

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✅ How to Fix the Issue (Production-Safe Solutions)

✔ Solution 1: Fix the DMN Conditions (Best & Fastest)

• Remove quotes from Boolean values

• Ensure conditions return true or false

Correct examples:

approved = true
amount > 1000
not(isValid)

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✔ Solution 2: Enforce Boolean at Process Level

Before calling DMN:

• Convert variables explicitly

Example (Java Service Task):

execution.setVariable(
  "approved",
  Boolean.parseBoolean(
    execution.getVariable("approved").toString()
  )
);

This ensures DMN receives a Boolean, not a String.

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✔ Solution 3: Validate Variables Before DMN Execution

Add a validation step:

• Script task

• Service task

Check:

• Variable existence

• Variable type

Fail early instead of breaking in DMN.

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✔ Solution 4: Fix REST Payloads

When starting processes via REST, send Booleans properly:

❌ Wrong:

"approved": { "value": "true" }

✔ Correct:

"approved": { "value": true }

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⚠️ Common Mistakes to Avoid

❌ Using "true" / "false" in DMN
❌ Assuming Camunda auto-converts types
❌ Skipping variable validation
❌ Testing only with Cockpit / Tasklist and not REST

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🏗️ Best Practices for Production

• Always define clear variable types

• Keep DMN logic simple and explicit

• Validate inputs before DMN

• Align:

  • REST payload

  • Process variables

  • DMN input types

• Test DMN with real production-like data

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🎯 Why This Happens Mostly in Production

In development:

• Variables often come from forms

• Values are controlled

In production:

• REST calls

• External systems

• Multiple integrations

➡️ Type mismatches become unavoidable unless explicitly handled.

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📌 Final Thoughts

Camunda DMN Boolean errors are not engine bugs.
They are almost always caused by type mismatches or incorrect FEEL expressions.

Once you enforce:

• Proper Boolean values

• Clean DMN conditions

• Early validation

These issues disappear completely.

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💼 Professional Support Available

If you are facing issues in real projects related to enterprise backend development or workflow automation, I provide paid consulting, production debugging, project support, and focused trainings.

Technologies covered include Java, Spring Boot, PL/SQL, Azure, and workflow automation (jBPM, Camunda BPM, RHPAM).

📧 Contact: ishikhanirankari@gmail.com | info@realtechnologiesindia.com

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