Fix typo in expand_derivatives

Author: YingboMa

src/differentials.jl

@@ -89,7 +89,7 @@ function expand_derivatives(O::Term, simplify=true; occurances=nothing)
             else
                 inner = expand_derivatives(D(o.args[1]), false)
                 # if the inner expression is not expandable either, return
-                if operation(inner) isa Differential
+                if inner isa Term && operation(inner) isa Differential
                     return O
                 else
                     return expand_derivatives(o.op(inner), simplify)