diff --git a/src/kyupy/sim.py b/src/kyupy/sim.py
index 9c90305..bc2926f 100644
--- a/src/kyupy/sim.py
+++ b/src/kyupy/sim.py
@@ -160,7 +160,7 @@ class SimOps:
     """A static scheduler that translates a Circuit into a topologically sorted list of basic logic operations (self.ops) and
     a memory mapping (self.c_locs, self.c_caps) for use in simulators.
 
-    :param circuit: The circuit to create a schedule for.
+    :param circuit: The circuit to create a schedule for. Must contain cells from KYUPY techlib only.
     :param strip_forks: If enabled, the scheduler will not include fork nodes to safe simulation time.
         Stripping forks will cause interconnect delay annotations of lines read by fork nodes to be ignored.
     :param c_reuse: If enabled, memory of intermediate signal waveforms will be re-used. This greatly reduces
@@ -201,8 +201,7 @@ class SimOps:
 
             for l in level_lines:
                 n = l.driver
-                if len(n.ins) > 4:
-                    log.warn(f'too many input pins: {n}')
+                assert len(n.ins) <= 4, f'Too many input pins for node {n}. Map circuit to KYUPY techlib first using resolce_tlib_cells().'
                 in_idxs = [n.ins[x].index if len(n.ins) > x and n.ins[x] is not None else self.zero_idx for x in [0,1,2,3]]
                 if n in ppio2idx:
                     in_idxs[0] = self.ppi_offset + ppio2idx[n]
@@ -226,10 +225,8 @@ class SimOps:
                                 if in_idxs[2] == self.zero_idx:
                                     sp = prims[2]
                             break
-                    if sp is None:
-                        log.warn(f'ignored cell of unknown type: {n}')
-                    else:
-                        level_ops.append((sp, l.index, *in_idxs, *a_ctrl[l]))
+                    assert sp is not None, f'Unsupported node type {n}. Map circuit to KYUPY techlib first using resolce_tlib_cells().'
+                    level_ops.append((sp, l.index, *in_idxs, *a_ctrl[l]))
 
             if len(level_ops) > 0: levels.append(level_ops)
             level_lines = prev_level_lines