diff --git a/kwant/continuum/_common.py b/kwant/continuum/_common.py
index 5fd97c6c4cf94b2f0726d019cac7ffa595f2f15e..fa2449399fe0771bdd539bbe565db127ba3fcdaa 100644
--- a/kwant/continuum/_common.py
+++ b/kwant/continuum/_common.py
@@ -25,6 +25,9 @@ import warnings
 
 from .._common import reraise_warnings
 
+# TODO: remove when sympy correctly includes MutableDenseMatrix (lol).
+sympy_classes = set(sympy_classes) | {sympy.MutableDenseMatrix}
+
 momentum_operators = sympy.symbols('k_x k_y k_z', commutative=False)
 position_operators = sympy.symbols('x y z', commutative=False)