diff --git a/src/strings.cpp b/src/strings.cpp
index 6e9665ad9e..5d0fa56753 100644
--- a/src/strings.cpp
+++ b/src/strings.cpp
@@ -191,8 +191,9 @@ void SetDParamStr(uint n, const char *str)
 	SetDParam(n, (uint64)(size_t)str);
 }
 
-void InjectDParam(int amount)
+void InjectDParam(uint amount)
 {
+	assert((uint)amount < lengthof(_decode_parameters));
 	memmove(_decode_parameters + amount, _decode_parameters, sizeof(_decode_parameters) - amount * sizeof(uint64));
 }
 
diff --git a/src/strings_func.h b/src/strings_func.h
index f83af7874f..2c53bac8c7 100644
--- a/src/strings_func.h
+++ b/src/strings_func.h
@@ -11,7 +11,7 @@ char *InlineString(char *buf, StringID string);
 char *GetString(char *buffr, StringID string, const char *last);
 const char *GetStringPtr(StringID string);
 
-void InjectDParam(int amount);
+void InjectDParam(uint amount);
 
 static inline void SetDParamX(uint64 *s, uint n, uint64 v)
 {