diff --git a/ecp5/pack.cc b/ecp5/pack.cc
index df139860..69d7ccf8 100644
--- a/ecp5/pack.cc
+++ b/ecp5/pack.cc
@@ -240,7 +240,7 @@ class Ecp5Packer
                     ci->disconnectPort(id_M);
                     ci->ports.erase(id_M);
                 }
-                if (di->driver.cell != nullptr && di->driver.cell->type == id_TRELLIS_COMB && di->driver.port == id_F) {
+                if (di && di->driver.cell != nullptr && di->driver.cell->type == id_TRELLIS_COMB && di->driver.port == id_F) {
                     CellInfo *comb = di->driver.cell;
                     if (comb->cluster != ClusterId()) {
                         // Special procedure where the comb cell is part of an existing macro