diff --git a/local.mk b/local.mk
index 6a7074e8e..6951c179e 100644
--- a/local.mk
+++ b/local.mk
@@ -1,6 +1,8 @@
 clean-files += Makefile.config
 
-GLOBAL_CXXFLAGS += -Wno-deprecated-declarations
+GLOBAL_CXXFLAGS += -Wno-deprecated-declarations -Werror=switch
+# Allow switch-enum to be overridden for files that do not support it, usually because of dependency headers.
+ERROR_SWITCH_ENUM = -Werror=switch-enum
 
 $(foreach i, config.h $(wildcard src/lib*/*.hh), \
   $(eval $(call install-file-in, $(i), $(includedir)/nix, 0644)))
diff --git a/mk/patterns.mk b/mk/patterns.mk
index 86a724806..c81150260 100644
--- a/mk/patterns.mk
+++ b/mk/patterns.mk
@@ -1,10 +1,10 @@
 $(buildprefix)%.o: %.cc
 	@mkdir -p "$(dir $@)"
-	$(trace-cxx) $(CXX) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cxx) $(CXX) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) $(ERROR_SWITCH_ENUM) -MMD -MF $(call filename-to-dep, $@) -MP
 
 $(buildprefix)%.o: %.cpp
 	@mkdir -p "$(dir $@)"
-	$(trace-cxx) $(CXX) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) -MMD -MF $(call filename-to-dep, $@) -MP
+	$(trace-cxx) $(CXX) -o $@ -c $< $(CPPFLAGS) $(GLOBAL_CXXFLAGS_PCH) $(GLOBAL_CXXFLAGS) $(CXXFLAGS) $($@_CXXFLAGS) $(ERROR_SWITCH_ENUM) -MMD -MF $(call filename-to-dep, $@) -MP
 
 $(buildprefix)%.o: %.c
 	@mkdir -p "$(dir $@)"
diff --git a/src/libexpr/local.mk b/src/libexpr/local.mk
index 2171e769b..d243b9cec 100644
--- a/src/libexpr/local.mk
+++ b/src/libexpr/local.mk
@@ -46,3 +46,5 @@ $(foreach i, $(wildcard src/libexpr/flake/*.hh), \
 $(d)/primops.cc: $(d)/imported-drv-to-derivation.nix.gen.hh $(d)/primops/derivation.nix.gen.hh $(d)/fetchurl.nix.gen.hh
 
 $(d)/flake/flake.cc: $(d)/flake/call-flake.nix.gen.hh
+
+src/libexpr/primops/fromTOML.o:	ERROR_SWITCH_ENUM =
diff --git a/src/libutil/logging.cc b/src/libutil/logging.cc
index 741e1d1f5..5a2dd99af 100644
--- a/src/libutil/logging.cc
+++ b/src/libutil/logging.cc
@@ -68,7 +68,7 @@ public:
             case lvlNotice: case lvlInfo: c = '5'; break;
             case lvlTalkative: case lvlChatty: c = '6'; break;
             case lvlDebug: case lvlVomit: c = '7';
-            default: c = '7'; break;
+            default: c = '7'; break; // should not happen, and missing enum case is reported by -Werror=switch-enum
             }
             prefix = std::string("<") + c + ">";
         }