diff --git a/odetoolbox/singularity_detection.py b/odetoolbox/singularity_detection.py
index 98013bdb..40e06655 100644
--- a/odetoolbox/singularity_detection.py
+++ b/odetoolbox/singularity_detection.py
@@ -101,6 +101,9 @@ def _is_matrix_defined_under_substitution(A: sympy.Matrix, cond_set: Set[Symmetr
             a set with equations, where the left-hand side of each equation is the variable that is to be subsituted, and the right-hand side is the expression to put in its place
         """
         for val in sympy.flatten(A):
+            if isinstance(val, float) or isinstance(val, int) or isinstance(val, sympy.core.numbers.Number):
+                continue
+
             expr_sub = val.copy()
             for eq in cond_set:
                 expr_sub = expr_sub.subs(eq.lhs, eq.rhs)