diff --git a/src/misc/dbg_helpers.h b/src/misc/dbg_helpers.h
index 670548d351..379b2e82e1 100644
--- a/src/misc/dbg_helpers.h
+++ b/src/misc/dbg_helpers.h
@@ -137,7 +137,7 @@ struct DumpTarget {
 
 	void WriteIndent();
 
-	void WriteLine(const char *format, ...);
+	void CDECL WriteLine(const char *format, ...) WARN_FORMAT(2, 3);
 	void WriteValue(const char *name, const char *value_str);
 	void WriteTile(const char *name, TileIndex t);
 
diff --git a/src/misc/str.hpp b/src/misc/str.hpp
index 8ef8e8f877..9a4663a6e7 100644
--- a/src/misc/str.hpp
+++ b/src/misc/str.hpp
@@ -100,7 +100,7 @@ struct CStrA : public CBlobT<char>
 	}
 
 	/** Add formated string (like sprintf) at the end of existing contents. */
-	int AddFormat(const char *format, ...)
+	int CDECL AddFormat(const char *format, ...) WARN_FORMAT(2, 3)
 	{
 		va_list args;
 		va_start(args, format);
@@ -110,7 +110,7 @@ struct CStrA : public CBlobT<char>
 	}
 
 	/** Assign formated string (like sprintf). */
-	int Format(const char *format, ...)
+	int CDECL Format(const char *format, ...) WARN_FORMAT(2, 3)
 	{
 		base::Free();
 		va_list args;