--- a/libmultipath/structs.h
+++ b/libmultipath/structs.h
@@ -106,7 +106,7 @@ enum yes_no_undef_states {
  * _FIND_MULTIPATHS_F must have the same value as YNU_YES.
  * Generate a compile time error if that isn't the case.
  */
-char ___error1___[-(_FIND_MULTIPATHS_F != YNU_YES)];
+extern char ___error1___[-(_FIND_MULTIPATHS_F != YNU_YES)];
 
 #define find_multipaths_on(conf) \
 	(!!((conf)->find_multipaths & _FIND_MULTIPATHS_F))
