diff --git a/xs/src/libslic3r/ConditionalGcode.cpp b/xs/src/libslic3r/ConditionalGcode.cpp
index ee28b7a7c..6bef678df 100644
--- a/xs/src/libslic3r/ConditionalGcode.cpp
+++ b/xs/src/libslic3r/ConditionalGcode.cpp
@@ -61,7 +61,7 @@ std::string evaluate(const std::string& expression_string) {
         #if SLIC3R_DEBUG
         std::cerr << __FILE__ << ":" << __LINE__ << " "<< "Failed to parse: " << expression_string.c_str() << std::endl;
         #endif
-        result << "" << expression_string << "";
+        result << "\x80" << expression_string << "\x81";
     }
     std::string output = result.str();
     trim(output);
diff --git a/xs/t/24_gcodemath.t b/xs/t/24_gcodemath.t
index 70663809e..8989b2b55 100644
--- a/xs/t/24_gcodemath.t
+++ b/xs/t/24_gcodemath.t
@@ -47,6 +47,6 @@ use Test::More tests => 7;
         my $test_string = "M104 S{a}; Sets temp to {4*5}";
 
         my $result = Slic3r::ConditionalGCode::apply_math($test_string);
-        is $result, "M104 Sa; Sets temp to 20", 'string (minus brackets) on failure to parse.';
+        is $result, "M104 S{a}; Sets temp to 20", 'string (minus brackets) on failure to parse.';
     }
 }