diff --git a/vvp/part.cc b/vvp/part.cc
index 7261cff1e..a14551205 100644
--- a/vvp/part.cc
+++ b/vvp/part.cc
@@ -239,17 +239,20 @@ vvp_fun_part_var::~vvp_fun_part_var()
 }
 
 bool vvp_fun_part_var::recv_vec4_(vvp_net_ptr_t port, const vvp_vector4_t&bit,
-                                  unsigned&base, vvp_vector4_t&source,
+                                  long&base, vvp_vector4_t&source,
                                   vvp_vector4_t&ref)
 {
-      unsigned long tmp;
+      long tmp;
       switch (port.port()) {
 	  case 0:
 	    source = bit;
 	    break;
 	  case 1:
-	    tmp = ULONG_MAX;
-	    vector4_to_value(bit, tmp);
+	      // LONG_MIN is before the vector and is used to
+	      // represent a 'bx value on the select input.
+	    tmp = LONG_MIN;
+	      // We need a new &PV<> that knows if the index is signed.
+	    vector4_to_value(bit, tmp, false);
 	    if (tmp == base) return false;
 	    base = tmp;
 	    break;
@@ -262,11 +265,11 @@ bool vvp_fun_part_var::recv_vec4_(vvp_net_ptr_t port, const vvp_vector4_t&bit,
       vvp_vector4_t res (wid_);
 
       for (unsigned idx = 0 ;  idx < wid_ ;  idx += 1) {
-	    unsigned adr = base+idx;
-	    if (adr >= source.size())
-		  break;
+	    long adr = base+idx;
+	    if (adr < 0) continue;
+	    if ((unsigned)adr >= source.size()) break;
 
-	    res.set_bit(idx, source.value(adr));
+	    res.set_bit(idx, source.value((unsigned)adr));
       }
 
       if (! ref.eeq(res)) {
@@ -311,7 +314,7 @@ void vvp_fun_part_var_sa::recv_vec4_pv(vvp_net_ptr_t port, const vvp_vector4_t&b
 struct vvp_fun_part_var_state_s {
       vvp_fun_part_var_state_s() : base(0) { }
 
-      unsigned base;
+      long base;
       vvp_vector4_t source;
       vvp_vector4_t ref;
 };
diff --git a/vvp/part.h b/vvp/part.h
index 687b76063..f42dc6cd3 100644
--- a/vvp/part.h
+++ b/vvp/part.h
@@ -130,7 +130,7 @@ class vvp_fun_part_var  : public vvp_net_fun_t {
 
     protected:
       bool recv_vec4_(vvp_net_ptr_t port, const vvp_vector4_t&bit,
-                      unsigned&base, vvp_vector4_t&source,
+                      long&base, vvp_vector4_t&source,
                       vvp_vector4_t&ref);
 
       unsigned wid_;
@@ -154,7 +154,7 @@ class vvp_fun_part_var_sa  : public vvp_fun_part_var {
                         vvp_context_t);
 
     private:
-      unsigned base_;
+      long base_;
       vvp_vector4_t source_;
 	// Save the last output, for detecting change.
       vvp_vector4_t ref_;
diff --git a/vvp/vpi_signal.cc b/vvp/vpi_signal.cc
index 6f5ec8d36..dfe454d9c 100644
--- a/vvp/vpi_signal.cc
+++ b/vvp/vpi_signal.cc
@@ -997,12 +997,24 @@ static int PV_get_base(struct __vpiPV*rfp)
 	/* If the width is zero then tbase is the constant. */
       if (rfp->twid == 0) return rfp->tbase;
 
+	/* Get the value from thread space. */
       int tval = 0;
       for (unsigned idx = 0 ;  idx < rfp->twid ;  idx += 1) {
 	    vvp_bit4_t bit = vthread_get_bit(vpip_current_vthread,
                                              rfp->tbase + idx);
-	    if (bit == BIT4_1) {
+	    switch (bit) {
+		case BIT4_X:
+		case BIT4_Z:
+		    /* We use INT_MIN to indicate an X base. */
+		  return INT_MIN;
+		  break;
+
+		case BIT4_1:
 		  tval |= 1<<idx;
+		  break;
+
+		case BIT4_0:
+		  break; // Do nothing!
 	    }
       }