diff --git a/src/V3Unknown.cpp b/src/V3Unknown.cpp
index 8c952c9a0..1a3dae823 100644
--- a/src/V3Unknown.cpp
+++ b/src/V3Unknown.cpp
@@ -137,6 +137,7 @@ private:
     virtual void visit(AstNodeModule* nodep) override {
         UINFO(4, " MOD   " << nodep << endl);
         VL_RESTORER(m_modp);
+        VL_RESTORER(m_constXCvt);
         {
             m_modp = nodep;
             m_constXCvt = true;
@@ -144,25 +145,34 @@ private:
         }
     }
     virtual void visit(AstAssignDly* nodep) override {
-        m_assigndlyp = nodep;
-        VL_DO_DANGLING(iterateChildren(nodep), nodep);  // May delete nodep.
-        m_assigndlyp = nullptr;
+        VL_RESTORER(m_assigndlyp);
+        {
+            m_assigndlyp = nodep;
+            VL_DO_DANGLING(iterateChildren(nodep), nodep);  // May delete nodep.
+        }
     }
     virtual void visit(AstAssignW* nodep) override {
-        m_assignwp = nodep;
-        VL_DO_DANGLING(iterateChildren(nodep), nodep);  // May delete nodep.
-        m_assignwp = nullptr;
+        VL_RESTORER(m_assignwp);
+        {
+            m_assignwp = nodep;
+            VL_DO_DANGLING(iterateChildren(nodep), nodep);  // May delete nodep.
+        }
     }
     virtual void visit(AstCaseItem* nodep) override {
-        m_constXCvt = false;  // Avoid losing the X's in casex
-        iterateAndNextNull(nodep->condsp());
-        m_constXCvt = true;
-        iterateAndNextNull(nodep->bodysp());
+        VL_RESTORER(m_constXCvt);
+        {
+            m_constXCvt = false;  // Avoid losing the X's in casex
+            iterateAndNextNull(nodep->condsp());
+            m_constXCvt = true;
+            iterateAndNextNull(nodep->bodysp());
+        }
     }
     virtual void visit(AstNodeDType* nodep) override {
-        m_constXCvt = false;  // Avoid losing the X's in casex
-        iterateChildren(nodep);
-        m_constXCvt = true;
+        VL_RESTORER(m_constXCvt);
+        {
+            m_constXCvt = false;  // Avoid losing the X's in casex
+            iterateChildren(nodep);
+        }
     }
     void visitEqNeqCase(AstNodeBiop* nodep) {
         UINFO(4, " N/EQCASE->EQ " << nodep << endl);
diff --git a/test_regress/t/t_math_countbits.v b/test_regress/t/t_math_countbits.v
index 8a1d9d4c5..e82c9ee93 100644
--- a/test_regress/t/t_math_countbits.v
+++ b/test_regress/t/t_math_countbits.v
@@ -37,6 +37,18 @@ module t(/*AUTOARG*/
    reg [5:0] result_59_3;
    reg [6:0] result_70_3;
 
+   initial begin
+      if ($countbits(32'b111100000000, '1) != 4) $stop;
+      if ($countbits(32'b111100000000, '0) != 28) $stop;
+      if ($countbits(32'b111100000000, '0, '1) != 32) $stop;
+      if ($countbits(4'bxxx0, 'x) != 3) $stop;
+      if ($countbits(4'bzzz0, 'z) != 3) $stop;
+      if ($countbits(4'b1zz0, 'z, '0) != 3) $stop;
+      if ($countbits(4'b1xx0, 'x, '0) != 3) $stop;
+      if ($countbits(4'b1xx0, 'x, '0, '1) != 4) $stop;
+      if ($countbits(4'bzzx0, 'x, 'z) != 3) $stop;
+   end
+
    always @* begin
       result_16_1 = $countbits(in16, ctrl0);
       result_16_2 = $countbits(in16, ctrl0, ctrl1);
@@ -73,11 +85,13 @@ module t(/*AUTOARG*/
          if ($countbits(32'b1100x01z101, 'x, 'z) != 2) $stop;
          if ($countbits(32'b1100x01z101, 'x, 'z, '1) != 7) $stop;
 
-`ifndef VERILATOR   // Unsup
          if ($countbits(val, '1) != 7) $stop;
          if ($countones(val) != 7) $stop;
          if ($countbits(val, '0) != 25) $stop;
          if ($countbits(val, '0, '1) != 32) $stop;
+`ifndef VERILATOR   // Unsup
+         if ($countbits(val, 'x) != 0) $stop;
+         if ($countbits(val, 'z) != 0) $stop;
          if ($countbits(val, 'x, 'z) != 0) $stop;
 `endif
       end