diff --git a/src/obj.h b/src/obj.h
index 2ac313c5..43b2cc56 100644
--- a/src/obj.h
+++ b/src/obj.h
@@ -78,7 +78,7 @@ const int _SIZEOF_VALUE = sizeof(_Value);
 #ifdef POCKETPY_H
 #define UNREACHABLE() throw std::runtime_error( "L" + std::to_string(__LINE__) + ": UNREACHABLE()! This should be a bug, please report it");
 #else
-#define UNREACHABLE() throw std::runtime_error( __FILE__ + ":" + std::to_string(__LINE__) + " UNREACHABLE()!");
+#define UNREACHABLE() throw std::runtime_error( __FILE__ + std::string(":") + std::to_string(__LINE__) + " UNREACHABLE()!");
 #endif
 
 struct PyObject {