diff --git a/src/scripting/thingdef_data.cpp b/src/scripting/thingdef_data.cpp
index 35d2733f5..a10d05710 100644
--- a/src/scripting/thingdef_data.cpp
+++ b/src/scripting/thingdef_data.cpp
@@ -1270,7 +1270,7 @@ DEFINE_ACTION_FUNCTION(FStringStruct, LastIndexOf)
 	PARAM_SELF_STRUCT_PROLOGUE(FString);
 	PARAM_STRING(substr);
 	PARAM_INT_DEF(endIndex);
-	ACTION_RETURN_INT(self->LastIndexOf(substr, endIndex));
+	ACTION_RETURN_INT(self->LastIndexOfBroken(substr, endIndex));
 }
 
 DEFINE_ACTION_FUNCTION(FStringStruct, RightIndexOf)
@@ -1278,7 +1278,7 @@ DEFINE_ACTION_FUNCTION(FStringStruct, RightIndexOf)
 	PARAM_SELF_STRUCT_PROLOGUE(FString);
 	PARAM_STRING(substr);
 	PARAM_INT_DEF(endIndex);
-	ACTION_RETURN_INT(self->RIndexOf(substr, endIndex));
+	ACTION_RETURN_INT(self->LastIndexOf(substr, endIndex));
 }
 
 DEFINE_ACTION_FUNCTION(FStringStruct, ToUpper)
diff --git a/src/zstring.cpp b/src/zstring.cpp
index e207cdb6b..5e57df45b 100644
--- a/src/zstring.cpp
+++ b/src/zstring.cpp
@@ -507,31 +507,11 @@ long FString::IndexOfAny (const char *charset, long startIndex) const
 	return long(brk - Chars);
 }
 
-long FString::LastIndexOf (const FString &substr) const
-{
-	return LastIndexOf (substr.Chars, long(Len()), substr.Len());
-}
-
-long FString::LastIndexOf (const char *substr) const
-{
-	return LastIndexOf (substr, long(Len()), strlen(substr));
-}
-
 long FString::LastIndexOf (char subchar) const
 {
 	return LastIndexOf (subchar, long(Len()));
 }
 
-long FString::LastIndexOf (const FString &substr, long endIndex) const
-{
-	return LastIndexOf (substr.Chars, endIndex, substr.Len());
-}
-
-long FString::LastIndexOf (const char *substr, long endIndex) const
-{
-	return LastIndexOf (substr, endIndex, strlen(substr));
-}
-
 long FString::LastIndexOf (char subchar, long endIndex) const
 {
 	if ((size_t)endIndex > Len())
@@ -548,8 +528,10 @@ long FString::LastIndexOf (char subchar, long endIndex) const
 	return -1;
 }
 
-long FString::LastIndexOf (const char *substr, long endIndex, size_t substrlen) const
+long FString::LastIndexOfBroken (const FString &_substr, long endIndex) const
 {
+	const char *substr = _substr.GetChars();
+	size_t substrlen = _substr.Len();
 	if ((size_t)endIndex > Len())
 	{
 		endIndex = long(Len());
@@ -596,27 +578,27 @@ long FString::LastIndexOfAny (const char *charset, long endIndex) const
 	return -1;
 }
 
-long FString::RIndexOf (const FString &substr) const
+long FString::LastIndexOf (const FString &substr) const
 {
-	return RIndexOf(substr.Chars, Len() - substr.Len(), substr.Len());
+	return LastIndexOf(substr.Chars, Len() - substr.Len(), substr.Len());
 }
 
-long FString::RIndexOf (const FString &substr, long endIndex) const
+long FString::LastIndexOf (const FString &substr, long endIndex) const
 {
-	return RIndexOf(substr.Chars, endIndex, substr.Len());
+	return LastIndexOf(substr.Chars, endIndex, substr.Len());
 }
 
-long FString::RIndexOf (const char *substr) const
+long FString::LastIndexOf (const char *substr) const
 {
-	return RIndexOf(substr, Len() - strlen(substr), strlen(substr));
+	return LastIndexOf(substr, Len() - strlen(substr), strlen(substr));
 }
 
-long FString::RIndexOf (const char *substr, long endIndex) const
+long FString::LastIndexOf (const char *substr, long endIndex) const
 {
-	return RIndexOf(substr, endIndex, strlen(substr));
+	return LastIndexOf(substr, endIndex, strlen(substr));
 }
 
-long FString::RIndexOf (const char *substr, long endIndex, size_t substrlen) const
+long FString::LastIndexOf (const char *substr, long endIndex, size_t substrlen) const
 {
 	if ((size_t)endIndex + substrlen > Len())
 	{
diff --git a/src/zstring.h b/src/zstring.h
index d61408dc7..67fef4a18 100644
--- a/src/zstring.h
+++ b/src/zstring.h
@@ -201,24 +201,21 @@ public:
 	long IndexOfAny (const FString &charset, long startIndex=0) const;
 	long IndexOfAny (const char *charset, long startIndex=0) const;
 
-	long LastIndexOf (const FString &substr) const;
-	long LastIndexOf (const char *substr) const;
+	// This is only kept for backwards compatibility with old ZScript versions that used this function and depend on its bug.
 	long LastIndexOf (char subchar) const;
-	long LastIndexOf (const FString &substr, long endIndex) const;
-	long LastIndexOf (const char *substr, long endIndex) const;
+	long LastIndexOfBroken (const FString &substr, long endIndex) const;
 	long LastIndexOf (char subchar, long endIndex) const;
-	long LastIndexOf (const char *substr, long endIndex, size_t substrlen) const;
 
 	long LastIndexOfAny (const FString &charset) const;
 	long LastIndexOfAny (const char *charset) const;
 	long LastIndexOfAny (const FString &charset, long endIndex) const;
 	long LastIndexOfAny (const char *charset, long endIndex) const;
 
-	long RIndexOf (const FString &substr) const;
-	long RIndexOf (const FString &substr, long endIndex) const;
-	long RIndexOf (const char *substr) const;
-	long RIndexOf (const char *substr, long endIndex) const;
-	long RIndexOf (const char *substr, long endIndex, size_t substrlen) const;
+	long LastIndexOf (const FString &substr) const;
+	long LastIndexOf (const FString &substr, long endIndex) const;
+	long LastIndexOf (const char *substr) const;
+	long LastIndexOf (const char *substr, long endIndex) const;
+	long LastIndexOf (const char *substr, long endIndex, size_t substrlen) const;
 
 	void ToUpper ();
 	void ToLower ();
diff --git a/wadsrc/static/zscript/base.txt b/wadsrc/static/zscript/base.txt
index 23ebafaf8..f310e2900 100644
--- a/wadsrc/static/zscript/base.txt
+++ b/wadsrc/static/zscript/base.txt
@@ -815,7 +815,7 @@ struct StringStruct native
 	native int CharCodeAt(int pos) const;
 	native String Filter();
 	native int IndexOf(String substr, int startIndex = 0) const;
-	deprecated("3.5") native int LastIndexOf(String substr, int endIndex = 2147483647) const;
+	deprecated("3.5.1") native int LastIndexOf(String substr, int endIndex = 2147483647) const;
 	native int RightIndexOf(String substr, int endIndex = 2147483647) const;
 	native void ToUpper();
 	native void ToLower();