diff --git a/src/scgenerator/parameter.py b/src/scgenerator/parameter.py
index fde6c64..14fbc56 100644
--- a/src/scgenerator/parameter.py
+++ b/src/scgenerator/parameter.py
@@ -15,7 +15,7 @@ import numpy as np
 from scgenerator.const import MANDATORY_PARAMETERS, __version__
 from scgenerator.evaluator import Evaluator, EvaluatorError
 from scgenerator.io import CustomEncoder, DataFile, custom_decode_hook
-from scgenerator.operators import Qualifier, SpecOperator
+from scgenerator.operators import Qualifier, SpecOperator, VariableQuantity
 
 T = TypeVar("T")
 DISPLAY_INFO = {}
@@ -409,7 +409,7 @@ class Parameters:
     worker_num: int = Parameter(positive(int))
 
     # computed
-    linear_operator: SpecOperator = Parameter(is_function, can_pickle=False)
+    linear_operator: VariableQuantity = Parameter(is_function, can_pickle=False)
     nonlinear_operator: SpecOperator = Parameter(is_function, can_pickle=False)
     conserved_quantity: Qualifier = Parameter(is_function, can_pickle=False)
     fft: Callable[[np.ndarray], np.ndarray] = Parameter(is_function, can_pickle=False)
diff --git a/src/scgenerator/plotting.py b/src/scgenerator/plotting.py
index 2407319..bd67f9a 100644
--- a/src/scgenerator/plotting.py
+++ b/src/scgenerator/plotting.py
@@ -1165,6 +1165,8 @@ def summary_plot(
 
     if z is None:
         z = np.arange(specs.shape[0])
+    elif len(z) > specs.shape[0]:
+        z = z[:specs.shape[0]]
 
     if wl_range is None:
         imin, imax = math.span_above(wl_int, wl_int.max() * 1e-6)