flowable/flowable-engine · error · IllegalArgumentException

Cannot coerce property to array index:

Error message

Cannot coerce property to array index: 

What it means

JsonNodeELResolver.toIndex throws IllegalArgumentException('Cannot coerce property to array index: ' + property) when the property object is neither a Number nor a String, so it cannot be interpreted as a JSON array index. Only numeric types and numeric strings are valid array indices here.

Solutions

1. Convert the property to a Number or numeric String before array access
2. Check the expression so the value inside [] is the intended integer variable, not an object/boolean
3. Cast or transform the variable (e.g. .intValue(), toString of a number) prior to indexing

Example fix

// before
${jsonArray[flag]}  // flag is Boolean
// after
${jsonArray[indexVar]}  // indexVar is Integer

if (!(prop instanceof Number) && !(prop instanceof String)) { throw new IllegalArgumentException("Array index must be Number or numeric String"); }

boolean isIndexable(Object p) { return p instanceof Number || (p instanceof String s && !s.isEmpty() && s.chars().allMatch(Character::isDigit)); }

try { expr.getValue(ctx); } catch (FlowableException e) { if (e.getCause() instanceof IllegalArgumentException iae && iae.getMessage().startsWith("Cannot coerce")) { throw new IllegalArgumentException("Array index must be numeric", iae); } throw e; }

When it happens

Trigger: Indexing a JSON array with a non-numeric, non-string object such as a Boolean, Map, or JsonNode, e.g. ${jsonArray[true]} or ${jsonArray[someObject]}.

Common situations: Accidentally passing a nested JSON node or POJO as the index; using a boolean flag variable as an index; template generation that interpolates the wrong variable into the brackets.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Cannot parse array index:
• Cannot coerce property to array index:
• Cannot convert value of type
• Could not set field
• Could not set field

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/3feedcdb38a09340. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine-common/src/main/java/org/flowable/common/engine/impl/el/JsonNodeELResolver.java:370

        } else {
            node.setNull(index);
        }

        context.setPropertyResolved(true);
    }

    protected int toIndex(Object property) {
        int index;
        if (property instanceof Number) {
            index = ((Number) property).intValue();
        } else if (property instanceof String) {
            try {
                index = Integer.valueOf((String) property);
            } catch (NumberFormatException e) {
                throw new IllegalArgumentException("Cannot parse array index: " + property, e);
            }
        } else {
            throw new IllegalArgumentException("Cannot coerce property to array index: " + property);
        }

        return index;
    }

    /**
     * Test whether the given base should be resolved by this ELResolver.
     * 
     * @param base
     *            The bean to analyze.
     * @return base != null
     */
    private final boolean isResolvable(Object base) {
        return JsonUtil.isJsonNode(base);
    }
}

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