diff --git a/configure b/configure
index 6564c8c26e4..b301ad73c81 100755
--- a/configure
+++ b/configure
@@ -534,7 +534,7 @@ opt optimize-llvm 1 "build optimized LLVM"
 opt optimize-tests 1 "build tests with optimizations"
 opt libcpp 1 "build with llvm with libc++ instead of libstdc++ when using clang"
 opt llvm-assertions 1 "build LLVM with assertions"
-opt debug-assertions 1 "build with extra debug fun"
+opt debug-assertions 0 "build with extra debug fun"
 opt fast-make 0 "use .gitmodules as timestamp for submodule deps"
 opt ccache 0 "invoke gcc/clang via ccache to reuse object files between builds"
 opt local-rust 0 "use an installed rustc rather than downloading a snapshot"
diff --git a/mk/main.mk b/mk/main.mk
index 4428dcc43c8..0c80c3f84f0 100644
--- a/mk/main.mk
+++ b/mk/main.mk
@@ -126,11 +126,11 @@ endif
 
 CFG_JEMALLOC_FLAGS += $(JEMALLOC_FLAGS)
 
-ifdef CFG_DISABLE_DEBUG_ASSERTIONS
-  CFG_RUSTC_FLAGS += --cfg ndebug
-else
+ifdef CFG_ENABLE_DEBUG_ASSERTIONS
   $(info cfg: enabling more debugging (CFG_ENABLE_DEBUG_ASSERTIONS))
   CFG_RUSTC_FLAGS += --cfg debug -C debug-assertions=on
+else
+  CFG_RUSTC_FLAGS += --cfg ndebug
 endif
 
 ifdef SAVE_TEMPS