微分方程式をPythonで解きたいのですが，dataframeエラーが出てしまいます

import sympy as sp
import math
import cv2
import numpy as np
import matplotlib.pyplot as plt
import csv
import os
import glob
import pandas as pd

in_excel_file = 'matome'
file = in_excel_file

pd.options.display.precision = 10
options = {}
options['strings_to_formulas'] = False
options['strings_to_urls'] = False
with pd.ExcelWriter('yre_z_DH=0.4_.xlsx', mode='w') as writer:
    results = []
    for i in range(35):
        print(i)
        def loadyre(file,i):  
            yre = pd.read_excel(file+'.xlsx', skiprows=4, usecols=[i+3], nrows=5, header=None)   #解析対象となるデータの読み込み
            return yre
        y0 = loadyre(file,i)
        print(y0)
        
        D = 0.016
        H = 0.04
        d = 0.015
        h = 0.039
        
        U = i*0.25 + 0.25
        print(U)
        
        π = math.pi
        m = 0.0156
        L = 0.3
        fc = 9.0
        ρ = 1.18
        M = (m/(ρ*H*D*L))*(D/H)
        Vr = U/(fc*H)
        A = H*L
        sp.init_printing()
        sp.var('z')
        y = sp.Function('y')(z) #yはzの関数(Function)
        E = 4.28*10*10*10*10*10*10
        I = (1/12)*(D*H*H*H-d*h*h*h)
        dCL = 0.3 #dCL/dα
        FL = -((ρ*U*A)/2*M)*dCL
        ML = FL/2
        a = ML/(E*I)
        eq = sp.Eq( y.diff(z,1) + a * y*y , 0 )
        yrez = sp.dsolve(eq)
        print(yrez)
        yrez2 = sp.dsolve(eq, ics={y.subs(z,0):y0})
        print(yrez2)
        results.append(yrez.subs(z, 190))

    df = pd.DataFrame(results).T
    
    df.to_excel(writer)

上記のコードを実行すると，以下のエラーが出てしまいます．
Excelからデータを読み込むこと，方程式を解くことを切り離して行うと，どちらともうまく回ります．方程式だけを解くときはy0に適当に値を入れました．

どのように改良すればよいでしょうか？

エラー文

0
              3
0  0.0000000058
0.25
Eq(y(z), -1/(C1 + 0.152757414958076*z))
Traceback (most recent call last):
  File "C:\Users\片持ち.py", line 56, in <module>
    yrez2 = sp.dsolve(eq, ics={y.subs(z,0):y0})
  File "C:\Users\Python\Python37\site-packages\sympy\solvers\ode\ode.py", line 631, in dsolve
    x0=x0, n=n, **kwargs)
  File "C:\Users\Python\Python37\site-packages\sympy\solvers\deutils.py", line 210, in _desolve
    n=terms, x0=x0, prep=prep)
  File "C:\Users\Python\Python37\site-packages\sympy\solvers\ode\ode.py", line 963, in classify_ode
    ics = sympify(ics)
  File "C:\Users\Python\Python37\site-packages\sympy\core\sympify.py", line 361, in sympify
    return conv(a)
  File "C:\Users\Python\Python37\site-packages\sympy\core\containers.py", line 310, in <lambda>
    converter[dict] = lambda d: Dict(*d.items())
  File "C:\Users\Python\Python37\site-packages\sympy\core\containers.py", line 234, in __new__
    elements = frozenset(items)
  File "C:\Users\Python\Python37\site-packages\sympy\core\containers.py", line 106, in __hash__
    return hash(self.args)
TypeError: unhashable type: 'DataFrame'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\片持ち.py", line 62, in <module>
    df.to_excel(writer)
  File "C:\Users\Python\Python37\site-packages\pandas\io\excel\_base.py", line 1020, in __exit__
    self.close()
  File "C:\Users\Python\Python37\site-packages\pandas\io\excel\_base.py", line 1024, in close
    content = self.save()
  File "C:\Users\Python\Python37\site-packages\pandas\io\excel\_openpyxl.py", line 80, in save
    self.book.save(self.handles.handle)
  File "C:\Users\Python\Python37\site-packages\openpyxl\workbook\workbook.py", line 407, in save
    save_workbook(self, filename)
  File "C:\Users\Python\Python37\site-packages\openpyxl\writer\excel.py", line 293, in save_workbook
    writer.save()
  File "C:\Users\Python\Python37\site-packages\openpyxl\writer\excel.py", line 275, in save
    self.write_data()
  File "C:\Users\Python\Python37\site-packages\openpyxl\writer\excel.py", line 89, in write_data
    archive.writestr(ARC_WORKBOOK, writer.write())
  File "C:\Users\Python\Python37\site-packages\openpyxl\workbook\_writer.py", line 148, in write
    self.write_views()
  File "C:\Users\Python\Python37\site-packages\openpyxl\workbook\_writer.py", line 135, in write_views
    active = get_active_sheet(self.wb)
  File "C:\Users\Python\Python37\site-packages\openpyxl\workbook\_writer.py", line 33, in get_active_sheet
    raise IndexError("At least one sheet must be visible")
IndexError: At least one sheet must be visible