diff --git a/src/smt/seq/seq_nielsen.cpp b/src/smt/seq/seq_nielsen.cpp
index 098f58518..810198963 100644
--- a/src/smt/seq/seq_nielsen.cpp
+++ b/src/smt/seq/seq_nielsen.cpp
@@ -310,7 +310,7 @@ namespace seq {
     }
 
     unsigned nielsen_node::var_ub(euf::snode* var) const {
-        SASSERT(!var);
+        SASSERT(var);
         unsigned v = UINT_MAX;
         m_var_ub.find(var->id(), v);
         return v;
diff --git a/src/smt/seq/seq_regex.cpp b/src/smt/seq/seq_regex.cpp
index 0a8eca302..c0597bd51 100644
--- a/src/smt/seq/seq_regex.cpp
+++ b/src/smt/seq/seq_regex.cpp
@@ -1173,7 +1173,8 @@ namespace seq {
         seq_util& seq = m_sg.get_seq_util();
         unsigned max_c = seq.max_char();
 
-        VERIFY(re_expr);
+        if (!re_expr)
+            return char_set::full(max_c);
 
         // full_char: the whole alphabet [0, max_char]
         if (seq.re.is_full_char(re_expr))
diff --git a/src/smt/seq_model.cpp b/src/smt/seq_model.cpp
index 724ac360d..256db7a91 100644
--- a/src/smt/seq_model.cpp
+++ b/src/smt/seq_model.cpp
@@ -324,6 +324,8 @@ namespace smt {
         SASSERT(sat_node);
         for (auto const& mem : sat_node->str_mems()) {
             SASSERT(mem.m_str && mem.m_regex);
+            if (mem.is_trivial())
+                continue; // empty string in nullable regex: already satisfied, no variable to constrain
             VERIFY(mem.is_primitive()); // everything else should have been eliminated already
             euf::snode* first = mem.m_str->first();
             unsigned id = first->id();