--- a/wx-config.in
+++ b/wx-config.in
@@ -973,7 +973,7 @@ fi
 
 is_monolithic() { [ "x@MONOLITHIC@" = "x1" ]; }
 is_static()     { [ -n "$this_linkage" ]; }
-is_installed()  { [ -z "$this_prefix" ]; }
+is_installed()  { [ "$this_prefix" ]; }
 
 
 # Is the user after a support utility?