diff --git a/validators/src/main/java/de/redsix/dmncheck/feel/FeelTypecheck.java b/validators/src/main/java/de/redsix/dmncheck/feel/FeelTypecheck.java
index 5c16276b..7f449882 100644
--- a/validators/src/main/java/de/redsix/dmncheck/feel/FeelTypecheck.java
+++ b/validators/src/main/java/de/redsix/dmncheck/feel/FeelTypecheck.java
@@ -68,7 +68,7 @@ private static Either<ValidationResult.Builder.ElementStep, ExpressionType> type
     }
 
     private static Either<ValidationResult.Builder.ElementStep, ExpressionType> typecheckUnaryExpression(final Context context, final Operator operator, final FeelExpression operand) {
-        final Stream<Operator> allowedOperators = Stream.of(Operator.GT, Operator.GE, Operator.LT, Operator.LE, Operator.NOT);
+        final Stream<Operator> allowedOperators = Stream.of(Operator.GT, Operator.GE, Operator.LT, Operator.LE, Operator.NOT, Operator.SUB);
         return typecheck(context, operand).bind(type ->
                     check(allowedOperators.anyMatch(operator::equals), "Operator is not supported in UnaryExpression.")
                     .orElse(checkOperatorCompatibility(type, operator))
diff --git a/validators/src/test/java/de/redsix/dmncheck/feel/FeelTypecheckTest.java b/validators/src/test/java/de/redsix/dmncheck/feel/FeelTypecheckTest.java
index 84815ace..a0d04036 100644
--- a/validators/src/test/java/de/redsix/dmncheck/feel/FeelTypecheckTest.java
+++ b/validators/src/test/java/de/redsix/dmncheck/feel/FeelTypecheckTest.java
@@ -90,7 +90,7 @@ void unboundVariableHasNoType() {
     }
 
     @ParameterizedTest
-    @CsvSource({"<5, INTEGER", "<5.2, DOUBLE"})
+    @CsvSource({"<5, INTEGER", "<5.2, DOUBLE", "-1, INTEGER"})
     void lessThanExpressionHasNumericType(String input, String expectedType) {
         final FeelExpression expression = FeelParser.PARSER.parse(input);
         final Either<ValidationResult.Builder.ElementStep, ExpressionType> type = FeelTypecheck.typecheck(expression);
diff --git a/validators/src/test/java/de/redsix/dmncheck/validators/OutputEntryTypeValidatorTest.java b/validators/src/test/java/de/redsix/dmncheck/validators/OutputEntryTypeValidatorTest.java
index 08bf6c47..c4afeb8f 100644
--- a/validators/src/test/java/de/redsix/dmncheck/validators/OutputEntryTypeValidatorTest.java
+++ b/validators/src/test/java/de/redsix/dmncheck/validators/OutputEntryTypeValidatorTest.java
@@ -21,7 +21,7 @@ class OutputEntryTypeValidatorTest extends WithDecisionTable {
     private final OutputEntryTypeValidator testee = new OutputEntryTypeValidator();
 
     @ParameterizedTest
-    @CsvSource({"integer, 42", "long, 42", "double, 42", "integer, "})
+    @CsvSource({"integer, 42", "long, 42", "double, 42", "integer, -1", "integer, "})
     void shouldAcceptWellTypedOutputExpression(final String typeref, final String textContent) {
         final Output output = modelInstance.newInstance(Output.class);
         output.setTypeRef(typeref);