Substitution using Dictionary with Matrix as Value

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As a newcomer to SAGE, trying to use it to do symbolic linear algebra, I am wondering why substitution of a variable using a dictionary doesn't work in this case:

sage: aMatrix = matrix(SR,1,1)
sage: var('aVariable')
aVariable
sage: aDict = {}
sage: aDict[aVariable] = aMatrix
sage: aDict[aVariable]
[0]

but:

sage: aVariable.subs(aDict)
....
/Applications/sage/local/lib/python2.7/site-packages/sage/symbolic/expression.so in sage.symbolic.expression.Expression.substitute (sage/symbolic/expression.cpp:16025)()

/Applications/sage/local/lib/python2.7/site-packages/sage/symbolic/expression.so in sage.symbolic.expression.Expression.coerce_in (sage/symbolic/expression.cpp:11265)()

/Applications/sage/local/lib/python2.7/site-packages/sage/structure/parent_old.so in sage.structure.parent_old.Parent._coerce_ (sage/structure/parent_old.c:3369)()

/Applications/sage/local/lib/python2.7/site-packages/sage/structure/parent.so in sage.structure.parent.Parent.coerce (sage/structure/parent.c:8912)()

TypeError: no canonical coercion from Full MatrixSpace of 1 by 1 dense matrices over Symbolic Ring to Symbolic Ring

Functionality to substitute matrices for variables seems to be indispensable to doing symbolic linear algebra, so I am sure there is a proper way to do this.