diff --git a/link_const.cc b/link_const.cc
index 9b01c75b3..0e8c7e39d 100644
--- a/link_const.cc
+++ b/link_const.cc
@@ -232,3 +232,58 @@ verinum Nexus::driven_vector() const
 
       return val;
 }
+
+/*
+ * Calculate a vector that represent all the bits of the vector, with
+ * each driven bit set to true, otherwise false.
+ */
+vector<bool> Nexus::driven_mask(void) const
+{
+      vector<bool> mask (vector_width());
+
+      for (const Link*cur = first_nlink() ; cur ; cur = cur->next_nlink()) {
+
+	    Link::DIR link_dir = cur->get_dir();
+	    if (link_dir==Link::PASSIVE)
+		  continue;
+	    if (link_dir==Link::INPUT)
+		  continue;
+
+	    const NetPins*obj = cur->get_obj();
+
+	      // If the link is to a variable (REG or INTEGER) then
+	      // the variable is driving all the bits. We have our
+	      // complete answer, mark all the bits as driven and
+	      // finish. Otherwise, we are not going to get new
+	      // information from this node, move on.
+	    if (const NetNet*sig = dynamic_cast<const NetNet*> (obj)) {
+		  NetNet::Type sig_type = sig->type();
+		  if (sig_type==NetNet::INTEGER || sig_type==NetNet::REG) {
+			for (size_t idx = 0 ; idx < mask.size() ; idx += 1)
+			      mask[idx] = true;
+			return mask;
+		  }
+		  continue;
+	    }
+
+	    const NetPartSelect*obj_ps = dynamic_cast<const NetPartSelect*>(obj);
+	    if (obj_ps && obj_ps->dir()==NetPartSelect::VP)
+		  continue;
+	    if (obj_ps && cur->get_pin()!=1)
+		  continue;
+	    if (obj_ps) {
+		  for (unsigned idx = 0 ; idx < obj_ps->width() ; idx += 1) {
+			size_t bit = idx + obj_ps->base();
+			ivl_assert(*obj, bit < mask.size());
+			mask[bit] = true;
+		  }
+		  continue;
+	    }
+
+	    for (size_t idx = 0 ; idx < mask.size() ; idx += 1)
+		  mask[idx] = true;
+	    return mask;
+      }
+
+      return mask;
+}
diff --git a/netlist.h b/netlist.h
index 13b61e0a4..1f5d4e9be 100644
--- a/netlist.h
+++ b/netlist.h
@@ -386,6 +386,11 @@ class Nexus {
       verinum::V driven_value() const;
       verinum driven_vector() const;
 
+	/* Return a mask of the bits of this vector that are
+	   driven. This is usually all false or all true, but in
+	   special cases it may be a blend. */
+      std::vector<bool> driven_mask(void)const;
+
 	/* The code generator sets an ivl_nexus_t to attach code
 	   generation details to the nexus. */
       ivl_nexus_t t_cookie() const { return t_cookie_; }
diff --git a/synth2.cc b/synth2.cc
index c028817af..169c216eb 100644
--- a/synth2.cc
+++ b/synth2.cc
@@ -621,6 +621,10 @@ bool NetCondit::synth_async(Design*des, NetScope*scope,
 			     << "MISSING TEST FOR CORRECTNESS OF THE BLEND!"
 			     << endl;
 		  }
+		  vector<bool>mask = statement_input.pin(idx).nexus()->driven_mask();
+		  for (size_t bit = mux_off ; bit < mux_off+mux_width ; bit += 1) {
+			ivl_assert(*this, mask[bit]==false);
+		  }
 		  connect(nex_out.pin(idx), statement_input.pin(idx));
 	    }