problem on solve function,the result is empty sym:0-by-1

i got some trouble when i run this script function, at the solve function the result is empty sym:0-by-1, i am relatively new in MatLab can anyone help me?

function[u_g] = viscosity(Pm) % viscosity() calculates viscosity of the gas at given reservoir % temperature and pressure.

M_air=[28.96;29;29.56;30;31;31.86;32;32.3;33;33.50];

Avg_Mol_Wt= [25.48;26;26.12;26.79;27;27.60;28;29;29.40;30];

spgr=Avg_Mol_Wt.\M_air;

Pm=[3800;3850;3400;3450;3500;3550;3600;3650;3700;3750]; %initial reservoir pressure

T=[660;680;720;740;760;780;800;820;840;860]; %Reservoir Temperature= 660 deg R (Literature)

u_uncorrected = ((1.709 * (10^(-5)-(2.062*10^(-6)*spgr))).*(T-460))+((8.118*10^(-3))-(6.15*10^(-3)*log10(spgr)));

yCO2 = 0.03;

yN2 = 0.02;

yH2S = 0.01;

u_CO2 = (yCO2*(((9.08*10^(-3))*(log10(spgr)))+ (6.24*10^(-3))));

u_N2 = (yN2*((8.48*10^(-3)*log10(spgr))+ (9.59*10^(-3))));

u_H2S = (yH2S*((8.49*10^(-3)*log10(spgr))+ (3.73*10^(-3))));

u1 = u_uncorrected + u_CO2 + u_N2 + u_H2S ;

Tpc = 168 + (325 * spgr) -(12.5*(spgr.^2));

Ppc = 677 + (15*spgr) -(37.5*(spgr.^2));

Tpr = T.\Tpc;

Ppr = Pm.\Ppc;

% Constants for viscosity relation

na=[1

1

1

1

1

1

1

1

1

1];

a0 = na.*-2.5;

a1 = na.*3;

a2 = na.*-0.29;

a3 = na.* 0.008;

a4 = na.*2.89;

a5 = na.* -3.5;

a6 = na.*0.36;

a7 = na.*-0.01;

a8 = na.*-0.8;

a9 = na.*1.4;

a10 = na.*-0.15;

a11 = na.*0.0044;

a12 = na.*0.084;

a13 = na.*-0.18;

a14 = na.*0.02;

a15 = na.*-0.0006;

syms ug;

con = a0+(a1.*Ppr)+(a2.*(Ppr.^2))+ (a3.*(Ppr.^3)) +(Tpr.*(a4+(a5.*Ppr))+(a6.*(Ppr.^2))+(a7.*(Ppr.^3))+((Tpr.^2).*(a8+(a9.*Ppr)))+(a10.*(Ppr.^2))+(a11.*(Ppr.^3)))+((Tpr.^3).*(a12+(a13.*Ppr)+(a14.*(Ppr.^2))+(a15.*(Ppr.^3))))

eqn = ((Tpr.*ug) == u1.*exp(con));

temp= solve(eqn,'ug')

u_g = double(temp);

end

the result

>> viscosity

con =

-0.5039

-0.5552

-0.5859

-0.6263

-0.6414

-0.6689

-0.7055

-0.7509

-0.7738

-0.8039

temp =

Empty sym: 0-by-1

ans =

0×1 empty double column vector