diff --git a/src/libslic3r/Config.cpp b/src/libslic3r/Config.cpp
index 037bd3500..91e74a032 100644
--- a/src/libslic3r/Config.cpp
+++ b/src/libslic3r/Config.cpp
@@ -713,11 +713,8 @@ bool DynamicConfig::read_cli(int argc, char** argv, t_config_option_keys* extra,
         // Look for the cli -> option mapping.
         const auto it = opts.find(token);
         if (it == opts.end()) {
-            printf("Warning: unknown option --%s\n", token.c_str());
-            // instead of continuing, return false to caller
-            // to stop execution and print usage
-            return false;
-            //continue;
+			boost::nowide::cerr << "Unknown option --" << token.c_str() << std::endl;
+			return false;
         }
         const t_config_option_key opt_key = it->second;
         const ConfigOptionDef &optdef = this->def()->options.at(opt_key);
@@ -725,8 +722,8 @@ bool DynamicConfig::read_cli(int argc, char** argv, t_config_option_keys* extra,
         // look for it in the next token.
         if (optdef.type != coBool && optdef.type != coBools && value.empty()) {
             if (i == (argc-1)) {
-                printf("No value supplied for --%s\n", token.c_str());
-                continue;
+				boost::nowide::cerr << "No value supplied for --" << token.c_str() << std::endl;
+                return false;
             }
             value = argv[++ i];
         }
@@ -759,7 +756,10 @@ bool DynamicConfig::read_cli(int argc, char** argv, t_config_option_keys* extra,
             static_cast<ConfigOptionString*>(opt_base)->value = value;
         } else {
             // Any scalar value of a type different from Bool and String.
-            this->set_deserialize(opt_key, value, false);
+            if (! this->set_deserialize(opt_key, value, false)) {
+				boost::nowide::cerr << "Invalid value supplied for --" << token.c_str() << std::endl;
+				return false;
+			}
         }
     }
     return true;
diff --git a/src/slic3r.cpp b/src/slic3r.cpp
index b60c5b1dd..3b301ec2a 100644
--- a/src/slic3r.cpp
+++ b/src/slic3r.cpp
@@ -523,6 +523,8 @@ bool CLI::setup(int argc, char **argv)
     // If any option is unsupported, print usage and abort immediately.
     t_config_option_keys opt_order;
     if (! m_config.read_cli(argc, argv, &m_input_files, &opt_order)) {
+		// Separate error message reported by the CLI parser from the help.
+		boost::nowide::cerr << std::endl;
         this->print_help();
 		return false;
     }
@@ -615,7 +617,7 @@ std::string CLI::output_filepath(const Model &model, IO::ExportFormat format) co
     };
     auto proposed_path = boost::filesystem::path(model.propose_export_file_name_and_path(ext));
     // use --output when available
-	std::string cmdline_param = m_config.opt_string("output", false);
+	std::string cmdline_param = m_config.opt_string("output");
     if (! cmdline_param.empty()) {
         // if we were supplied a directory, use it and append our automatically generated filename
         boost::filesystem::path cmdline_path(cmdline_param);