diff --git i/xs/src/libslic3r/Config.hpp w/xs/src/libslic3r/Config.hpp
index 49e999b..d9b65d8 100644
--- i/xs/src/libslic3r/Config.hpp
+++ w/xs/src/libslic3r/Config.hpp
@@ -65,7 +65,7 @@ class ConfigOptionFloat : public ConfigOption
     
     bool deserialize(std::string str) {
         std::istringstream iss(str);
-        return iss >> this->value;
+        return bool(iss >> this->value);
     };
 };
 
@@ -124,7 +124,7 @@ class ConfigOptionInt : public ConfigOption
     
     bool deserialize(std::string str) {
         std::istringstream iss(str);
-        return iss >> this->value;
+        return bool(iss >> this->value);
     };
 };
 
@@ -249,7 +249,7 @@ class ConfigOptionPercent : public ConfigOption
     bool deserialize(std::string str) {
         // don't try to parse the trailing % since it's optional
         std::istringstream iss(str);
-        return iss >> this->value;
+        return bool(iss >> this->value);
     };
 };
 
@@ -279,7 +279,7 @@ class ConfigOptionFloatOrPercent : public ConfigOption
     bool deserialize(std::string str) {
         this->percent = str.find_first_of("%") != std::string::npos;
         std::istringstream iss(str);
-        return iss >> this->value;
+        return bool(iss >> this->value);
     };
 };