diff --git a/CMakeLists.txt b/CMakeLists.txt
index 417c3dd54..7d9dff233 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -13,6 +13,15 @@ PROJECT(fish)
 SET(CMAKE_CXX_STANDARD 11)
 SET(DEFAULT_BUILD_TYPE "RelWithDebInfo")
 
+# Use the default flags (#6296) but remove -DNDEBUG so that asserts remain enabled.
+STRING(REPLACE "-DNDEBUG" ""
+  CMAKE_CXX_FLAGS_RELWITHDEBINFO
+  "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
+
+STRING(REPLACE "-DNDEBUG" ""
+  CMAKE_CXX_FLAGS_RELEASE
+  "${CMAKE_CXX_FLAGS_RELEASE}")
+
 IF(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
     MESSAGE(STATUS "Setting build type to default '${DEFAULT_BUILD_TYPE}'")
     SET(CMAKE_BUILD_TYPE "${DEFAULT_BUILD_TYPE}")