diff --git a/include/cairo/context.hpp b/include/cairo/context.hpp
index e8f54188..af4807b4 100644
--- a/include/cairo/context.hpp
+++ b/include/cairo/context.hpp
@@ -165,7 +165,7 @@ namespace cairo {
 
       string utf8 = t.contents;
       string_util::unicode_charlist chars;
-      bool success = string_util::utf8_to_ucs4(utf8.c_str(), chars);
+      bool success = string_util::utf8_to_ucs4(utf8, chars);
 
       if (!success) {
         m_log.warn("Dropping invalid UTF8 text '%s'", utf8);
diff --git a/include/utils/string.hpp b/include/utils/string.hpp
index 328ef897..3f8ed30a 100644
--- a/include/utils/string.hpp
+++ b/include/utils/string.hpp
@@ -87,7 +87,7 @@ string utf8_truncate(string&& value, size_t len);
 /**
  * @brief Create a UCS-4 codepoint from a utf-8 encoded string
  */
-[[nodiscard]] bool utf8_to_ucs4(const char* src, unicode_charlist& result_list);
+[[nodiscard]] bool utf8_to_ucs4(const string& src, unicode_charlist& result_list);
 
 /**
  * @brief Convert a UCS-4 codepoint to a utf-8 encoded string
diff --git a/src/utils/string.cpp b/src/utils/string.cpp
index a66e00cb..54325b4a 100644
--- a/src/utils/string.cpp
+++ b/src/utils/string.cpp
@@ -280,10 +280,10 @@ static pair<int, uint32_t> utf8_get_len(uint8_t leading) {
  *
  * @return Whether the string is completely valid utf8
  */
-bool utf8_to_ucs4(const char* src, unicode_charlist& result_list) {
-  assert(src);
+bool utf8_to_ucs4(const string& src, unicode_charlist& result_list) {
+  result_list.reserve(src.size());
   bool has_errors = false;
-  const auto* begin = reinterpret_cast<const uint8_t*>(src);
+  const auto* begin = reinterpret_cast<const uint8_t*>(src.c_str());
 
   const auto* current = begin;
   while (*current) {
diff --git a/tests/unit_tests/utils/string.cpp b/tests/unit_tests/utils/string.cpp
index e0f1257e..428e648f 100644
--- a/tests/unit_tests/utils/string.cpp
+++ b/tests/unit_tests/utils/string.cpp
@@ -188,7 +188,7 @@ TEST_P(Utf8ToUCS4AsciiTest, correctness) {
   string_util::unicode_charlist result_list{};
   string str = GetParam();
 
-  bool success = string_util::utf8_to_ucs4(str.c_str(), result_list);
+  bool success = string_util::utf8_to_ucs4(str, result_list);
   ASSERT_TRUE(success);
 
   ASSERT_EQ(str.size(), result_list.size());
@@ -229,7 +229,7 @@ TEST_P(Utf8ToUCS4SingleTest, correctness) {
   string_util::unicode_charlist result_list{};
   const auto [str, codepoint] = GetParam();
 
-  bool success = string_util::utf8_to_ucs4(str.c_str(), result_list);
+  bool success = string_util::utf8_to_ucs4(str, result_list);
   ASSERT_TRUE(success);
 
   ASSERT_EQ(1, result_list.size());
@@ -251,7 +251,7 @@ const vector<string> utf8_to_ucs4_invalid_list = {
     "\xe0",         // 3 byte code point with only leading byte
     "\xf0",         // 4 byte code point with only leading byte
     "\xf0\x80\x80", // 4 byte code point with only 3 bytes
-    "\xe0\x70\x80", // 3 byte code point, 2nd byte has no continuation prefix
+    "\xe0\xf0\x80", // 3 byte code point, 2nd byte has no continuation prefix
 };
 
 INSTANTIATE_TEST_SUITE_P(Inst, Utf8ToUCS4InvalidTest, testing::ValuesIn(utf8_to_ucs4_invalid_list));
@@ -262,7 +262,7 @@ INSTANTIATE_TEST_SUITE_P(Inst, Utf8ToUCS4InvalidTest, testing::ValuesIn(utf8_to_
 TEST_P(Utf8ToUCS4InvalidTest, correctness) {
   string_util::unicode_charlist result_list{};
   const auto str = GetParam();
-  bool success = string_util::utf8_to_ucs4(str.c_str(), result_list);
+  bool success = string_util::utf8_to_ucs4(str, result_list);
   EXPECT_FALSE(success);
   EXPECT_EQ(0, result_list.size());
 }