diff --git a/src/libslic3r/Config.hpp b/src/libslic3r/Config.hpp
index 844287efb..7d96f6cf3 100644
--- a/src/libslic3r/Config.hpp
+++ b/src/libslic3r/Config.hpp
@@ -93,7 +93,7 @@ enum ConfigOptionMode {
     comExpert
 };
 
-enum PrinterTechnology
+enum PrinterTechnology : unsigned char
 {
     // Fused Filament Fabrication
     ptFFF,
diff --git a/src/slic3r/Utils/UndoRedo.hpp b/src/slic3r/Utils/UndoRedo.hpp
index 00ab55822..96b539f18 100644
--- a/src/slic3r/Utils/UndoRedo.hpp
+++ b/src/slic3r/Utils/UndoRedo.hpp
@@ -12,6 +12,7 @@
 namespace Slic3r {
 
 class Model;
+enum PrinterTechnology : unsigned char;
 
 namespace GUI {
 	class Selection;