diff --git a/conftest.py b/conftest.py
index 5a1bc481b9..03193c4212 100644
--- a/conftest.py
+++ b/conftest.py
@@ -287,6 +287,21 @@ def pytest_runtest_makereport(item, call):
             result.outcome = 'passed'
 
 
+def pytest_make_parametrize_id(config, val, argname):
+    """
+    Prevents pytest to make obsucre parameter names (param0, param1, ...)
+    and default to str(val) instead for better log readability.
+    """
+    # First see if it has a name
+    if hasattr(val, '__name__'):
+        return val.__name__
+    # Then try str(val)
+    try:
+        return str(val)
+    except Exception:
+        return None  # Fall back to default behavior
+
+
 # A list of optimization options/pipelines to be used in testing
 # regarding spatial and/or temporal blocking.
 opts_tiling = ['advanced',
diff --git a/devito/finite_differences/derivative.py b/devito/finite_differences/derivative.py
index 782132c4d8..e2ad8229c7 100644
--- a/devito/finite_differences/derivative.py
+++ b/devito/finite_differences/derivative.py
@@ -162,7 +162,8 @@ def _validate_expr(expr):
             try:
                 expr = diffify(expr)
             except Exception as e:
-                raise ValueError("`expr` must be a `Differentiable` type object") from e
+                d = type(expr)
+                raise ValueError(f"`expr` must be a `Differentiable` not {d}") from e
         return expr
 
     @staticmethod
diff --git a/devito/passes/iet/engine.py b/devito/passes/iet/engine.py
index e68a2101cf..519d9e4c25 100644
--- a/devito/passes/iet/engine.py
+++ b/devito/passes/iet/engine.py
@@ -223,6 +223,7 @@ def create_call_graph(root, efuncs):
     """
     dag = DAG(nodes=[root])
     queue = [root]
+    defuncs = {root}
 
     while queue:
         caller = queue.pop(0)
@@ -230,6 +231,7 @@ def create_call_graph(root, efuncs):
 
         for callee in filter_ordered([i.name for i in callees]):
             if callee in efuncs:  # Exclude foreign Calls, e.g., MPI calls
+                defuncs.add(callee)
                 try:
                     dag.add_node(callee)
                     queue.append(callee)
@@ -239,7 +241,7 @@ def create_call_graph(root, efuncs):
                 dag.add_edge(callee, caller)
 
     # Sanity check
-    assert dag.size == len(efuncs)
+    assert dag.size == len(defuncs)
 
     return dag