fitting - Keeping Nonlinear Model Fit Real

So I am using the NonlinearModelFit (NLM) command with a fairly simple function. There are 4 unknowns and the (x, y) points that I am fitting the function to. I am getting my approximations from a Manipulate so I can just tweak the values till it's close enough. But once I get the values into the NLM, I get this error about the function value not being a real number at these values (not the same values that I put into the NLM). I have tried several different sets of values and every time I get the same error. I do have two constraints that keep two of my values positive.

R = 0.42; Sigma = 73.06967052; Theta=1.32757;

nData={{0.989939, 4.62}, {0.989939, 4.64}, {0.989939, 4.66},
       {0.989939, 4.68}, {0.989939, 4.7}, {0.989939, 4.72},
       {0.989939, 4.74}, {0.989939, 4.76}, {0.989939, 4.78},
       {0.989939, 4.8}, {0.99398, 4.82}, {0.99398, 4.84},
       {0.99398, 4.86}, {0.99398, 4.88}, {0.99398, 4.9},
       {0.99398, 4.92}, {0.99398, 4.94}, {0.99398, 4.96},
       {0.99398, 4.98}, {0.99398, 5}, {0.99398, 5.02},
       {0.99398, 5.04}, {0.99398, 5.06}, {0.99398, 5.08},

       {0.99398, 5.1}};

nlm = NonlinearModelFit[
    nData, 
    {((3 + s)/(1 + s) 1/R (h + dh)^(1 + s))* ((p/(Sigma*Cos[Theta]))^s)*
        Hypergeometric2F1[1 + s, s, 2 + s, (h + dh)/h0], s > 0, p > 0}, 
    {{p, 0.207}, {s, 1}, {h0, 2.04}, {dh, 0.9}}, h
 ]

Comments

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