diff --git a/src/univ/themes/gtk.cpp b/src/univ/themes/gtk.cpp
index 99d48189d6..73ff1f5322 100644
--- a/src/univ/themes/gtk.cpp
+++ b/src/univ/themes/gtk.cpp
@@ -54,7 +54,7 @@
 // constants (to be removed, for testing only)
 // ----------------------------------------------------------------------------
 
-static const wxCoord BORDER_THICKNESS = 1;
+static const size_t BORDER_THICKNESS = 1;
 
 // ----------------------------------------------------------------------------
 // wxGTKRenderer: draw the GUI elements in GTK style