diff --git a/src/common/String.cpp b/src/common/String.cpp
index a197d17a63..41bbe01361 100644
--- a/src/common/String.cpp
+++ b/src/common/String.cpp
@@ -5,7 +5,7 @@
 
 #include "String.h"
 
-std::string ByteString::ConversionError::formatError(ByteString::value_type const *at, ByteString::value_type const *upto)
+ByteString ConversionError::formatError(ByteString::value_type const *at, ByteString::value_type const *upto)
 {
 	std::stringstream ss;
 	ss << "Could not convert sequence to UTF-8:";
@@ -77,7 +77,7 @@ ByteString String::ToUtf8() const
 		}
 		else if(result == std::codecvt_base::error)
 		{
-			throw ByteString::ConversionError(true);
+			throw ConversionError(true);
 		}
 		else if(result == std::codecvt_base::partial)
 		{
diff --git a/src/common/String.h b/src/common/String.h
index 76825f684a..706f270e0f 100644
--- a/src/common/String.h
+++ b/src/common/String.h
@@ -99,14 +99,6 @@ class ByteString : public std::basic_string<char>
 
 public:
 
-	class ConversionError : public std::runtime_error
-	{
-		static std::string formatError(value_type const *at, value_type const *upto);
-	public:
-		inline ConversionError(value_type const *at, value_type const *upto): std::runtime_error(formatError(at, upto)) {}
-		inline ConversionError(bool to): std::runtime_error(to ? "Could not convert to UTF-8" : "Could not convert from UTF-8") {}
-	};
-
 	String FromUtf8(bool ignoreError = true) const;
 	inline String FromAscii() const;
 
@@ -243,4 +235,13 @@ inline ByteString String::ToAscii() const
 		destination[i] = ByteString::value_type(operator[](i));
 	return destination;
 }
+
+class ConversionError : public std::runtime_error
+{
+	static ByteString formatError(ByteString::value_type const *at, ByteString::value_type const *upto);
+public:
+	inline ConversionError(ByteString::value_type const *at, ByteString::value_type const *upto): std::runtime_error(formatError(at, upto)) {}
+	inline ConversionError(bool to): std::runtime_error(to ? "Could not convert to UTF-8" : "Could not convert from UTF-8") {}
+};
+
 #endif