diff --git a/test/webfuse/mocks/mock_provider_client.hpp b/test/webfuse/mocks/mock_provider_client.hpp
index 2db3127..45fdf91 100644
--- a/test/webfuse/mocks/mock_provider_client.hpp
+++ b/test/webfuse/mocks/mock_provider_client.hpp
@@ -11,7 +11,7 @@ namespace webfuse_test
     class ProviderClientException: public std::runtime_error
     {
         public:
-            ProviderClientException(wf_status error_code);
+            explicit ProviderClientException(wf_status error_code);
             wf_status GetErrorCode();
         private:
             wf_status error_code_;