diff --git a/feature_flags_test.go b/feature_flags_test.go
index e5a5cea..790553b 100644
--- a/feature_flags_test.go
+++ b/feature_flags_test.go
@@ -27,6 +27,28 @@ func TestMatchPropertyValue(t *testing.T) {
 	}
 
 }
+
+
+func TestMatchPropertyInvalidOperator(t *testing.T) {
+	property := Property{
+		Key:      "Browser",
+		Value:    "Chrome",
+		Operator: "is_unknown",
+	}
+
+	properties := NewProperties().Set("Browser", "Chrome")
+
+	isMatch, err := matchProperty(property, properties)
+
+	if isMatch == true {
+		t.Error("Should not match")
+	}
+
+	if _, ok := err.(*InconclusiveMatchError); !ok {
+		t.Error("Error type is not a match")
+	}
+
+}
 func TestMatchPropertySlice(t *testing.T) {
 
 	property := Property{
diff --git a/featureflags.go b/featureflags.go
index 23b2914..012d1de 100644
--- a/featureflags.go
+++ b/featureflags.go
@@ -531,7 +531,8 @@ func matchProperty(property Property, properties Properties) (bool, error) {
 		return overrideValueOrderable <= valueOrderable, nil
 	}
 
-	return false, nil
+	return false, &InconclusiveMatchError{"Unknown operator: " + operator}
+
 }
 
 func validateOrderable(firstValue interface{}, secondValue interface{}) (float64, float64, error) {