diff --git a/src/scripting/thingdef_data.cpp b/src/scripting/thingdef_data.cpp
index a90cb3566..540b61a36 100644
--- a/src/scripting/thingdef_data.cpp
+++ b/src/scripting/thingdef_data.cpp
@@ -1264,7 +1264,8 @@ DEFINE_ACTION_FUNCTION(FStringStruct, LastIndexOf)
 {
 	PARAM_SELF_STRUCT_PROLOGUE(FString);
 	PARAM_STRING(substr);
-	ACTION_RETURN_INT(self->LastIndexOf(substr));
+	PARAM_INT_DEF(endIndex);
+	ACTION_RETURN_INT(self->LastIndexOf(substr, endIndex));
 }
 
 DEFINE_ACTION_FUNCTION(FStringStruct, ToUpper)
diff --git a/wadsrc/static/zscript/base.txt b/wadsrc/static/zscript/base.txt
index a6866f0ff..9cba0cb09 100644
--- a/wadsrc/static/zscript/base.txt
+++ b/wadsrc/static/zscript/base.txt
@@ -688,7 +688,7 @@ struct StringStruct native
 	native int CharCodeAt(int pos) const;
 	native String Filter();
 	native int IndexOf(String substr, int startIndex = 0) const;
-	native int LastIndexOf(String substr) const;
+	native int LastIndexOf(String substr, int endIndex = 2147483647) const;
 	native void ToUpper();
 	native void ToLower();
 	native int ToInt(int base = 0) const;