Having trouble in solving two differential equations using desolve_system

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I am trying to solve the following two differential equations simultaneously: $$Ma^2\frac{dM}{dr}+(M^2a+6a)\frac{da}{dr}+\frac{1}{r^2}=0$$ $$ar\frac{dM}{dr}+7Mr\frac{da}{dr}+2Ma=0$$ where $M=M(r)$ and $a=a(r)$ are the variables.

I had written the following code in Sage:

sage: r = var('r')
sage: M = function('M')(r)
sage: a = function('a')(r)
sage: de1 = (M*a*a*diff(M,r) + (M*M*a+6*a)*diff(a,r) + 1/(r*r) == 0)
sage: de2 = (a*r*diff(M,r) + 7*M*r*diff(a,r) + 2*M*a == 0)
sage: desolve_system([de1,de2], [M,a])

After writing the above code in Sage, I am getting the following error:

TypeError                                 Traceback (most recent call last)
<ipython-input-26-6bce8159491d> in <module>()
----> 1 desolve_system([de1, de2], [M,a])

/usr/lib/sagemath/local/lib/python2.7/site-packages/sage/calculus/desolvers.py in desolve_system(des, vars, ics, ivar)
    822         for dvar, ic in zip(dvars, ics[1:]):
    823             dvar.atvalue(ivar==ivar_ic, ic)
--> 824     soln = dvars[0].parent().desolve(des, dvars)
    825     if str(soln).strip() == 'false':
    826         raise NotImplementedError("Maxima was unable to solve this system.")

/usr/lib/sagemath/local/lib/python2.7/site-packages/sage/interfaces/interface.py in __call__(self, *args, **kwds)
    606 
    607     def __call__(self, *args, **kwds):
--> 608         return self._parent.function_call(self._name, list(args), kwds)
    609 
    610     def _sage_doc_(self):

/usr/lib/sagemath/local/lib/python2.7/site-packages/sage/interfaces/interface.py in function_call(self, function, args, kwds)
    532                                        [s.name() for s in args],
    533                                        ['%s=%s'%(key,value.name()) for key, value in kwds.items()])
--> 534         return self.new(s)
    535 
    536     def _function_call_string(self, function, args, kwds):

/usr/lib/sagemath/local/lib/python2.7/site-packages/sage/interfaces/interface.py in new(self, code)
    307 
    308     def new(self, code):
--> 309         return self(code)
    310 
    311     ###################################################################

/usr/lib/sagemath/local/lib/python2.7/site-packages/sage/interfaces/interface.py in __call__(self, x, name)
    242 
    243         if isinstance(x, six.string_types):
--> 244             return cls(self, x, name=name)
    245         try:
    246             return self._coerce_from_special_method(x)

/usr/lib/sagemath/local/lib/python2.7/site-packages/sage/interfaces/interface.py in __init__(self, parent, value, is_name, name)
    670                 self._name = parent._create(value, name=name)
    671             except (TypeError, RuntimeError, ValueError) as x:
--> 672                 raise TypeError(x)
    673 
    674     def _latex_(self):

TypeError: ECL says: Error executing code in Maxima: desolve: can't handle this case.

Can someone help me with the problem. I am new to Sage and so I could not interpret the error.

Thanks in advance!