diff --git a/gtsam/inference/Factor-inl.h b/gtsam/inference/Factor-inl.h
index 59db42e29..09336186e 100644
--- a/gtsam/inference/Factor-inl.h
+++ b/gtsam/inference/Factor-inl.h
@@ -47,10 +47,10 @@ namespace gtsam {
 		// Check that keys are all unique
 		std::multiset < Key > nonunique(keys_.begin(), keys_.end());
 		std::set < Key > unique(keys_.begin(), keys_.end());
-		bool correct = (nonunique.size() != unique.size())
-		&& std::equal(nonunique.begin(), nonunique.end(), unique.begin());
-		if (!correct)
-			throw std::logic_error("Factor::assertInvariants: detected inconsistency");
+		bool correct = (nonunique.size() == unique.size())
+				&& std::equal(nonunique.begin(), nonunique.end(), unique.begin());
+		if (!correct) throw std::logic_error(
+				"Factor::assertInvariants: detected inconsistency");
 #endif
 	}