diff --git a/test/testsuite.cpp b/test/testsuite.cpp
index 38851f64a..8b37ecac1 100644
--- a/test/testsuite.cpp
+++ b/test/testsuite.cpp
@@ -33,7 +33,7 @@ std::ostringstream output;
  **/
 
 struct CompareFixtures {
-    bool operator()(const TestFixture* lhs, const TestFixture* rhs) {
+    bool operator()(const TestFixture* lhs, const TestFixture* rhs) const {
         return lhs->classname < rhs->classname;
     }
 };