use "evaluate" to calculate UDF

[ck_123]

hi,i got one UDF named"Evaluatestr" used to perform calculation for formula in "string" format,but the UDF return #value! error if formula contain other UDF,to make thing simple and easy to understand, i had attached simple udf code and excel file in here

Option Explicit

Function Evaluatestr(RefCell As String)

'Application.Volatile

Evaluatestr = Evaluate(RefCell)

End Function

Option Explicit

Function udf(a As Range, b As Double, c As Double, d As Double, e As Double)

'Application.Volatile

udf = Evaluate(" sum(offset(" & a.Address(External:=True) & ",(" & b & "),(" & c & "),(" & d & "),(" & e & "))) ")

End Function

here ,the result:
1 SUM(OFFSET(A1,0,0,2,1))+1 7
2 UDF(A1,0,0,2,1)+1 #VALUE!


any solution for this?