junit-team/junit5 · error · ArgumentConversionException

Cannot convert null to ${targetClass.getName()}; consider se

Error message

Cannot convert null to ${targetClass.getName()}; consider setting 'nullable = true'

What it means

Thrown by JavaTimeArgumentConverter.convert() when the input is null and the @JavaTimeConversionPattern annotation does not have nullable = true set. This converter is used when a parameter is annotated with @JavaTimeConversionPattern to parse date/time values. Unlike the default converter, null is rejected unless explicitly opted in.

Source

Thrown at junit-jupiter-params/src/main/java/org/junit/jupiter/params/converter/JavaTimeArgumentConverter.java:62

		queries.put(LocalDateTime.class, LocalDateTime::from);
		queries.put(LocalTime.class, LocalTime::from);
		queries.put(OffsetDateTime.class, OffsetDateTime::from);
		queries.put(OffsetTime.class, OffsetTime::from);
		queries.put(Year.class, Year::from);
		queries.put(YearMonth.class, YearMonth::from);
		queries.put(ZonedDateTime.class, ZonedDateTime::from);
		TEMPORAL_QUERIES = Collections.unmodifiableMap(queries);
	}

	@Override
	protected @Nullable Object convert(@Nullable Object input, Class<?> targetClass,
			JavaTimeConversionPattern annotation) {

		if (input == null) {
			if (annotation.nullable()) {
				return null;
			}
			throw new ArgumentConversionException(
				"Cannot convert null to " + targetClass.getName() + "; consider setting 'nullable = true'");
		}
		TemporalQuery<?> temporalQuery = TEMPORAL_QUERIES.get(targetClass);
		if (temporalQuery == null) {
			throw new ArgumentConversionException("Cannot convert to " + targetClass.getName() + ": " + input);
		}
		String pattern = annotation.value();
		DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);
		return formatter.parse(input.toString(), temporalQuery);
	}

}

View on GitHub (pinned to 956246301e)

Solutions

Add nullable = true to the @JavaTimeConversionPattern annotation
Remove @NullSource / @NullAndEmptySource if null values are not expected for this parameter
Ensure CSV columns for date parameters are never empty

Example fix

// before
@ParameterizedTest
@NullSource
void test(@JavaTimeConversionPattern("yyyy-MM-dd") LocalDate date) { }

// after
@ParameterizedTest
@NullSource
void test(@JavaTimeConversionPattern(value = "yyyy-MM-dd", nullable = true) LocalDate date) { }

Defensive patterns

Strategy: validation

Validate before calling

// Check the annotation before using null-providing sources:
JavaTimeConversionPattern ann = parameter.getAnnotation(JavaTimeConversionPattern.class);
if (ann != null && !ann.nullable() && usesNullSource) {
    throw new IllegalStateException(
        "Set nullable = true on @JavaTimeConversionPattern or remove @NullSource");
}

Prevention

Set nullable = true on @JavaTimeConversionPattern when the source may provide null
Remove @NullSource / @NullAndEmptySource from tests with @JavaTimeConversionPattern parameters if null is not expected
Ensure CSV date columns are never empty when nullable is false

When it happens

Trigger: A @ParameterizedTest parameter annotated with @JavaTimeConversionPattern("yyyy-MM-dd") receiving a null argument from @NullSource, @NullAndEmptySource, or an empty CSV field, without setting nullable = true on the annotation.

Common situations: Adding @NullSource to a parameterized test that uses @JavaTimeConversionPattern for date parsing, without enabling nullable. CSV sources with empty date columns where the converter receives null. Combining null-testing sources with temporal type parameters.

Related errors

AI-assisted analysis of junit-team/junit5@956246301e (2026-08-04). Data as JSON: /data/errors/8c113c56f31781a3.json. Report an issue: GitHub.