初学者，自定义函数老是提示错误

刚刚学到自定义函数，试着编了如下的编码：
Private Sub Command1_Click()
Dim A, B, C, F As Single
F = Func1(A, B, C)
Text4 = F
End Sub
Private Sub Form_Load()
Dim A, B, C As Single
A = Val(Text1.Text)
B = Val(Text2.Text)
C = Val(Text3.Text)
End Sub
Public Function Func1(ByVal A As Single, B As Single, C As Single) As Single
Func1 = ((A - B) ^ 2 + C ^ 2) / ((A + B) ^ 2 + C ^ 2)
End Function
就是在三个文本框输入三个任意值，计算后根据函数在text4中显示，老是提示溢出，求高手指点，谢谢！