diff --git a/tgt-vhdl/stmt.cc b/tgt-vhdl/stmt.cc
index e7619b205..62e5f2206 100644
--- a/tgt-vhdl/stmt.cc
+++ b/tgt-vhdl/stmt.cc
@@ -138,7 +138,8 @@ static int draw_nbassign(vhdl_process *proc, stmt_container *container,
       // then use the uninitialized signal value.
       // The second test ensures that we only try to initialise
       // internal signals not ports
-      if (proc->is_initial() && ivl_signal_port(sig) == IVL_SIP_NONE) {
+      if (proc->is_initial() && ivl_signal_port(sig) == IVL_SIP_NONE
+          && rhs->constant()) {
          decl->set_initial(rhs);
       }
       else {
@@ -181,9 +182,10 @@ static int draw_assign(vhdl_process *proc, stmt_container *container,
          return 1;
       vhdl_expr *rhs = rhs_raw->cast(decl->get_type());
 
-      // As with non-blocking assignment, push assignments into the
-      // initialisation if we can
-      if (proc->is_initial() && ivl_signal_port(sig) == IVL_SIP_NONE) {
+      // As with non-blocking assignment, push constant assignments
+      // into the initialisation if we can
+      if (proc->is_initial() && ivl_signal_port(sig) == IVL_SIP_NONE
+          && rhs->constant()) {
          decl->set_initial(rhs);
       }
       else {
diff --git a/tgt-vhdl/vhdl_syntax.cc b/tgt-vhdl/vhdl_syntax.cc
index 314ee5ee0..bc1e6044f 100644
--- a/tgt-vhdl/vhdl_syntax.cc
+++ b/tgt-vhdl/vhdl_syntax.cc
@@ -585,7 +585,7 @@ void vhdl_assign_stmt::emit(std::ofstream &of, int level) const
 }
 
 vhdl_const_bits::vhdl_const_bits(const char *value, int width)   
-   : vhdl_expr(vhdl_type::nsigned(width))
+   : vhdl_expr(vhdl_type::nsigned(width), true)
 {
    // Can't rely on value being NULL-terminated
    while (width--)
diff --git a/tgt-vhdl/vhdl_syntax.hh b/tgt-vhdl/vhdl_syntax.hh
index 12c8a1336..66a892639 100644
--- a/tgt-vhdl/vhdl_syntax.hh
+++ b/tgt-vhdl/vhdl_syntax.hh
@@ -29,13 +29,16 @@ class vhdl_arch;
 
 class vhdl_expr : public vhdl_element {
 public:
-   vhdl_expr(vhdl_type* type) : type_(type) {}
+   vhdl_expr(vhdl_type* type, bool isconst=false)
+      : type_(type), isconst_(isconst) {}
    virtual ~vhdl_expr();
 
    const vhdl_type *get_type() const { return type_; }
+   bool constant() const { return isconst_; }
    virtual vhdl_expr *cast(const vhdl_type *to);
 private:
    vhdl_type *type_;
+   bool isconst_;
 };
 
 
@@ -114,7 +117,7 @@ private:
 class vhdl_const_string : public vhdl_expr {
 public:
    vhdl_const_string(const char *value)
-      : vhdl_expr(vhdl_type::string()), value_(value) {}
+      : vhdl_expr(vhdl_type::string(), true), value_(value) {}
 
    void emit(std::ofstream &of, int level) const;
 private:
@@ -134,7 +137,7 @@ private:
 class vhdl_const_bit : public vhdl_expr {
 public:
    vhdl_const_bit(char bit)
-      : vhdl_expr(vhdl_type::std_logic()), bit_(bit) {}
+      : vhdl_expr(vhdl_type::std_logic(), true), bit_(bit) {}
    void emit(std::ofstream &of, int level) const;
 private:
    char bit_;
@@ -147,7 +150,7 @@ enum time_unit_t {
 class vhdl_const_time : public vhdl_expr {
 public:
    vhdl_const_time(int64_t value, time_unit_t units)
-      : vhdl_expr(vhdl_type::time()), value_(value), units_(units) {}
+      : vhdl_expr(vhdl_type::time(), true), value_(value), units_(units) {}
    void emit(std::ofstream &of, int level) const;
 private:
    int64_t value_;
@@ -157,7 +160,7 @@ private:
 class vhdl_const_int : public vhdl_expr {
 public:
    vhdl_const_int(int64_t value)
-      : vhdl_expr(vhdl_type::integer()), value_(value) {}
+      : vhdl_expr(vhdl_type::integer(), true), value_(value) {}
    void emit(std::ofstream &of, int level) const;
 private:
    int64_t value_;