Index: lib/libkvm/kvm.c
===================================================================
--- lib/libkvm/kvm.c
+++ lib/libkvm/kvm.c
@@ -64,7 +64,7 @@
 
 #include "kvm_private.h"
 
-SET_DECLARE(kvm_arch, struct kvm_arch);
+SET_DECLARE_WEAK(kvm_arch, struct kvm_arch);
 
 static char _kd_is_null[] = "";
 
Index: sys/dev/uart/uart_cpu_fdt.h
===================================================================
--- sys/dev/uart/uart_cpu_fdt.h
+++ sys/dev/uart/uart_cpu_fdt.h
@@ -46,7 +46,7 @@
  * If your UART driver implements uart_class and custom device layer,
  * then use UART_FDT_CLASS for its declaration
  */
-SET_DECLARE(uart_fdt_class_set, struct ofw_compat_data );
+SET_DECLARE_WEAK(uart_fdt_class_set, struct ofw_compat_data );
 #define UART_FDT_CLASS(data)				\
 	DATA_SET(uart_fdt_class_set, data)
 
Index: sys/sys/linker_set.h
===================================================================
--- sys/sys/linker_set.h
+++ sys/sys/linker_set.h
@@ -84,7 +84,12 @@
 /*
  * Initialize before referring to a given linker set.
  */
-#define SET_DECLARE(set, ptype)					\
+#define SET_DECLARE(set, ptype)				\
+	extern ptype *__CONCAT(__start_set_,set);	\
+	extern ptype *__CONCAT(__stop_set_,set)
+
+/* Like SET_DECLARE() but also marks the symbols weak to support empty sets. */
+#define SET_DECLARE_WEAK(set, ptype)				\
 	extern ptype __weak_symbol *__CONCAT(__start_set_,set);	\
 	extern ptype __weak_symbol *__CONCAT(__stop_set_,set)