diff --git a/resis/ResReadSim.c b/resis/ResReadSim.c
index da55e54b..0320c3a9 100644
--- a/resis/ResReadSim.c
+++ b/resis/ResReadSim.c
@@ -426,6 +426,7 @@ ResSimSubckt(line)
 	return 1;
     }
     ttype = extGetDevType(EFDevTypes[j]);
+    ASSERT(ttype >= 0, "ttype<0");
 
     /* Find the device record that corresponds to the device name */
     for (devptr = ExtCurStyle->exts_device[ttype]; devptr; devptr = devptr->exts_next)