Hey, This is my first post in the Wolfram Community, and I must say, I have been looking up multiple discussions before, but I could not find an answer to the problem I'm having right now. I have made a function (S_d), which consists out of multiple functions for certain x-values, I have used a piecewise for this. When I call the function with a certain x-value (T-value in this case), I get the y-value from it. However, when I tried it again with a different T-value, I did not get my y-value, but the following: 
The following is what happens earlier in the notebook, which is what I expected and what I need in order to continue with the calculations: 
I would really appreciate if you could take a look at this and tell me where I made a mistake or what I can do to repair this and prevent this for future use. Thanks a lot in advance!
Best regards, Hidde Lansink
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