diff --git a/pydiso/mkl_solver.py b/pydiso/mkl_solver.py
index 46c47c3..d7259ce 100644
--- a/pydiso/mkl_solver.py
+++ b/pydiso/mkl_solver.py
@@ -295,7 +295,7 @@ def _validate_csr_matrix(self, mat):
 
         if mat.format != 'csr':
             warnings.warn(
-                "Converting %s matrix to CSR format."% A.__class__.__name__,
+                "Converting %s matrix to CSR format."% mat.__class__.__name__,
                 PardisoTypeConversionWarning,
                 stacklevel=3
             )