diff --git a/src/string.cc b/src/string.cc
index d9689938..682a6947 100644
--- a/src/string.cc
+++ b/src/string.cc
@@ -138,37 +138,37 @@ int str_to_int(StringView str)
     throw runtime_error{str + " is not a number"};
 }
 
-InplaceString<16> to_string(int val)
+InplaceString<15> to_string(int val)
 {
-    InplaceString<16> res;
+    InplaceString<15> res;
     res.m_length = sprintf(res.m_data, "%i", val);
     return res;
 }
 
-InplaceString<24> to_string(size_t val)
+InplaceString<23> to_string(size_t val)
 {
-    InplaceString<24> res;
+    InplaceString<23> res;
     res.m_length = sprintf(res.m_data, "%zu", val);
     return res;
 }
 
-InplaceString<24> to_string(Hex val)
+InplaceString<23> to_string(Hex val)
 {
-    InplaceString<24> res;
+    InplaceString<23> res;
     res.m_length = sprintf(res.m_data, "%zx", val.val);
     return res;
 }
 
-InplaceString<24> to_string(float val)
+InplaceString<23> to_string(float val)
 {
-    InplaceString<24> res;
+    InplaceString<23> res;
     res.m_length = sprintf(res.m_data, "%f", val);
     return res;
 }
 
-InplaceString<8> to_string(Codepoint c)
+InplaceString<7> to_string(Codepoint c)
 {
-    InplaceString<8> res;
+    InplaceString<7> res;
     char* ptr = res.m_data;
     utf8::dump(ptr, c);
     res.m_length = (int)(ptr - res.m_data);
diff --git a/src/string.hh b/src/string.hh
index c853ebe4..6970e74a 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -263,21 +263,23 @@ Optional<int> str_to_int_ifp(StringView str);
 template<size_t N>
 struct InplaceString
 {
-    constexpr operator StringView() const { return {m_data, m_length}; }
-    operator String() const { return {m_data, m_length}; }
+    static_assert(N < 256, "InplaceString cannot handle sizes >= 256");
 
-    ByteCount m_length;
+    constexpr operator StringView() const { return {m_data, ByteCount{m_length}}; }
+    operator String() const { return {m_data, ByteCount{m_length}}; }
+
+    unsigned char m_length;
     char m_data[N];
 };
 
 struct Hex { size_t val; };
 inline Hex hex(size_t val) { return {val}; }
 
-InplaceString<16> to_string(int val);
-InplaceString<24> to_string(size_t val);
-InplaceString<24> to_string(Hex val);
-InplaceString<24> to_string(float val);
-InplaceString<8>  to_string(Codepoint c);
+InplaceString<15> to_string(int val);
+InplaceString<23> to_string(size_t val);
+InplaceString<23> to_string(Hex val);
+InplaceString<23> to_string(float val);
+InplaceString<7>  to_string(Codepoint c);
 
 template<typename RealType, typename ValueType>
 decltype(to_string(std::declval<ValueType>()))