diff --git a/passes/equiv/equiv_opt.cc b/passes/equiv/equiv_opt.cc
index 6cdbd6717..df09510c3 100644
--- a/passes/equiv/equiv_opt.cc
+++ b/passes/equiv/equiv_opt.cc
@@ -74,16 +74,16 @@ struct EquivOptPass:public ScriptPass
 		log("\n");
 	}
 
-	std::string command, techmap_opts, make_opts;
-	bool assert, undef, multiclock, async2sync, nocheck, post;
+	std::string command, techmap_opts, make_opts, induct_opts;
+	bool assert, multiclock, async2sync, nocheck, post;
 
 	void clear_flags() override
 	{
 		command = "";
 		techmap_opts = "";
 		make_opts = "";
+		induct_opts = "";
 		assert = false;
-		undef = false;
 		multiclock = false;
 		async2sync = false;
 		nocheck = false;
@@ -118,7 +118,11 @@ struct EquivOptPass:public ScriptPass
 				continue;
 			}
 			if (args[argidx] == "-undef") {
-				undef = true;
+				induct_opts += " -undef";
+				continue;
+			}
+			if (args[argidx] == "-ignore-unknown-cells") {
+				induct_opts += " -ignore-unknown-cells";
 				continue;
 			}
 			if (args[argidx] == "-nocheck") {
@@ -213,11 +217,10 @@ struct EquivOptPass:public ScriptPass
 				opts = make_opts;
 			run("equiv_make" + opts + " gold gate equiv");
 			if (help_mode)
-				run("equiv_induct [-undef] equiv");
-			else if (undef)
-				run("equiv_induct -undef equiv");
+				opts = " [-undef] [-ignore-unknown-cells]";
 			else
-				run("equiv_induct equiv");
+				opts = induct_opts;
+			run("equiv_induct" + opts + " equiv");
 			if (help_mode)
 				run("equiv_status [-assert] equiv");
 			else if (assert)
diff --git a/tests/silimate/fanoutbuf.ys b/tests/silimate/fanoutbuf.ys
index 3fd70ef0a..38df60d7b 100644
--- a/tests/silimate/fanoutbuf.ys
+++ b/tests/silimate/fanoutbuf.ys
@@ -315,13 +315,13 @@ module top (
   wire intermediate1, intermediate2;
   assign intermediate1 = a & b;
   assign intermediate2 = intermediate1 | c;
-  
+
   assign w1 = intermediate1;
   assign w2 = intermediate1;
   assign w3 = intermediate1;
   assign w4 = intermediate1;
   assign w5 = intermediate1;
-  
+
   assign x1 = intermediate2;
   assign x2 = intermediate2;
   assign x3 = intermediate2;
@@ -376,7 +376,7 @@ EOF
 check -assert
 
 # Check equivalence after fanoutbuf - should insert buffers in sub module
-equiv_opt -assert fanoutbuf
+equiv_opt -assert -ignore-unknown-cells fanoutbuf
 design -load postopt
 select -assert-count 1 t:$buf
 select -assert-none t:$pos