diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index b592b719a..381339768 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -256,6 +256,7 @@ bool zstring::contains(zstring const& other) const {
 
 int zstring::indexof(zstring const& other, int offset) const {
     SASSERT(offset >= 0);
+    if (static_cast<unsigned>(offset) <= length() && other.length() == 0) return offset;
     if (static_cast<unsigned>(offset) == length()) return -1;
     if (other.length() + offset > length()) return -1;
     unsigned last = length() - other.length();