diff --git a/platform/qt/src/string_stdlib.cpp b/platform/qt/src/string_stdlib.cpp
index 635a01352..334091223 100644
--- a/platform/qt/src/string_stdlib.cpp
+++ b/platform/qt/src/string_stdlib.cpp
@@ -9,13 +9,13 @@ namespace mbgl {
 namespace platform {
 
 std::string uppercase(const std::string& str) {
-    auto upper = QString::fromUtf8(str.c_str()).toUpper().toUtf8();
+    auto upper = QString::fromUtf8(str.data(), str.length()).toUpper().toUtf8();
 
     return std::string(upper.constData(), upper.size());
 }
 
 std::string lowercase(const std::string& str) {
-    auto lower = QString::fromUtf8(str.c_str()).toLower().toUtf8();
+    auto lower = QString::fromUtf8(str.data(), str.length()).toLower().toUtf8();
 
     return std::string(lower.constData(), lower.size());
 }