diff --git a/src/ast/sls/sls_seq_plugin.cpp b/src/ast/sls/sls_seq_plugin.cpp
index 85f1b5289..3f09f3792 100644
--- a/src/ast/sls/sls_seq_plugin.cpp
+++ b/src/ast/sls/sls_seq_plugin.cpp
@@ -694,7 +694,7 @@ namespace sls {
         hashtable<zstring, zstring_hash_proc, default_eq<zstring>> set;
         set.insert(zstring(""));
         for (unsigned i = 0; i < val_other.length(); ++i) {
-            for (unsigned j = val_other.length(); j-- > 0; ) {
+            for (unsigned j = val_other.length() - i; j-- > 0; ) {
                 zstring sub = val_other.extract(i, j);
                 if (set.contains(sub))
                     break;
diff --git a/src/util/parray.h b/src/util/parray.h
index bc3c4adbe..1a51a8b56 100644
--- a/src/util/parray.h
+++ b/src/util/parray.h
@@ -414,6 +414,7 @@ public:
     void push_back(ref & r, value const & v) {
         if (r.m_ref == nullptr)
             mk(r);
+        SASSERT(r.m_ref);
         if (r.root()) {
             if (r.unshared()) {
                 rpush_back(r.m_ref, v);